I thought "east" meant "from the OTL Brazilian core colony" via the Amazon region too, and I think this has more potential actually.
We have recently rediscovered the various civilizations of the Amazonian region, and it is evident enough now that these collapsed due to diseases spread by early European expeditions up the Amazon.
But suppose there is an ATL variant on one of these Amazonian cultivation-based and major earthworks creating Amazonian peoples, who happen to be centered pretty far down the Amazon main course (though not actually a coastal people) who happen by a chance to survive whatever bugs it was that laid them all low OTL, and happen to come into close contact with the Portuguese. Enough people surviving the witch's brew of Old World diseases to sustain high density civilization in the Amazon region might be asking for something close to or literally ASB of course, but if we can roll with it somehow, then could Portuguese interactions with the remnant native civilization (and hence perhaps with remnants of other peoples who suffered the population implosion they did ATL, but now have a choice of places to emigrate to as refugees) and the Native American polity winds up being at least nominally Christianized but with its surviving political elites playing a near peer role with Portuguese emissaries and forces, so it is more of an alliance than a conquest.
If this can happen early enough, then the Native people of the eastern downriver polity will probably know something about the Andean situation themselves, via trade, and refugees from various devastated rainforest cultivator peoples from rather west/upriver near the Andes might know a lot more. A joint expedition under nominal Portuguese leadership but with Amazonian client/allies forming a major part of the total force might be far better able to navigate up an appropriate river route to get pretty close to Peru, then march the rest of the way with refugee Native guides from the region.
It lacks the tit for tat turning the tables on the Spanish arguments their explorations and claims in the wrong hemisphere are justified by their having arrived there from the Atlantic and going westbound from there with a reciprocal Portuguese west to east trans-Pacific expedition, but I certainly agree such an expedition might seem feasible in hindsight, what with Pizarro initially having under 200 men...but in reality at this early stage of world exploration by Europeans, there just isn't enough knowledge of the Pacific to allow such an expedition to arrive by anything but the wildest extremes of good fortune; as a plan it is an expensive form of suicide.
Aside from not being the OP at all, and the unlikely nature of the ATL Amazonian situation I suggest including the speculative possibility of cooperation between surviving Natives and the Portuguese, the big problem here is that Portugal getting hegemony over even part of the Andean empire (I am pretty sure the Spanish would be able to annex at least the northern portion of it) is just a footnote to the momentous ATL consequences of the Amazonians surviving. I would think any surviving civ in the Amazon basin would have very unique trade goods to offer, and some of the South American rainforest crops might be suitable for transplantation to similar climates elsewhere--some of which are in regions the Portuguese are already involved in--such as the central African rainforests for instance. And while the Spanish might be able to hold on to ill-gotten claim jumps in Nusantara, beyond the Philippines, I don't think they'd be in a good position to expel the Portuguese from there either, not entirely.
So Portugal, to the extent they can co-opt rather than attempt to tyrannize over the Amazonian complex and whatever bits of Peru they get, can perhaps pull in quite a substantial revenue from these two South American equatorial fiefdoms despite a lighter hand and production and governmental administration being done by Native clients, and on top of that augment their OTL trading revenues with ATL trade goods. A knock on might be experience with this sort of collaborative imperialism working for them in South America to enable more effective power projection into Central Africa, perhaps enabling them to get control of the southern African coast, and maybe more effective at expanding their sphere of control and influence in Southeast Asia and India too.
It is probably not possible for Portugal to rival Spain as an equal power in Europe, but perhaps between stealing some of the revenue that went to Spanish coffers OTL and thus weakening Spain somewhat, meanwhile acquiring these ATL assets gives them some extra leverage in Europe with revenues from the ATL additional trade volume in tropical crop derived goods, both in South America and in plantations elsewhere. Of course Spain and other European rival powers can also grow and trade in those goods as well, but the Portuguese have some advantage due to good relations with the people in South America who "invented" these crops. If the Christianization of those South Americans goes strongly enough and relations are decent the Portuguese can draw on their manpower in their rivalries elsewhere in the world too, to an increasing extent.
Perhaps this has further knock ons. For instance consider the Tudor dynasty in England; OTL Henry VII sought an alliance with Spain involving the arrangement of the marriage of Catherine of Aragon to the heir to the English throne, which was originally his older son Arthur, but then Arthur died and they switched over to Henry who became Henry VIII. What if in the ATL circumstances with an early Portuguese hegemony reaching all the way to the Andean altiplano and perhaps to the Pacific coast, Henry VII looks to Portugal instead? Obviously if "butterflies" from early changes are in play Arthur might not die in the sinking of the "White Ship" and become the English king, and perhaps married not to Catherine but some Portuguese princess instead, which of course completely changes the Tudor succession.
Also, England and Portugal have tended to fair to good relations OTL; if this factor continues in the ATL, England could be an early ally of Portugal versus Spain and be rewarded with Portuguese acceptance of English traders and colonies along their routes, or even actively soliciting the English to please set up bases of their own to assist the Portuguese in these zones. Meanwhile the Treaty of Tordesillas, instead of being somewhat weakened (and OTL none of the rival European nations outside Iberia who had potential global interests ever accepted the thing as something to respect) is pretty well trashed by the OP proposed ATL events. Portugal herself is in a weak position to challenge Spain in the Caribbean and Mesoamerica, to be sure--in a somewhat better position than OTL to challenge Spain's grip on the northern Caribbean regions of South America, but not a great one. But any Spanish objections to English colonization of North America far north of the Caribbean could be somewhat countered by Portugal giving their blessing to ventures in the region of the St Lawrence estuary, New England--indeed if the Iberian rivalry is hot enough, the Portuguese would be quite pleased to see their English allies intrude into Florida and grab as much as they can manage in the Caribbean. They might also actively support English colonization south of OTL Brazilian core settlements in OTL Argentina including Patagonia, and maybe Chile to cover the south flank of the approaches to any Pacific coast they reach overland.
Things would change radically with the Protestant reformation of course--either Portugal also goes some kind of Protestant, which strikes me as extremely unlikely and sure to embroil them in really severe wars with Spain which definitely won't, and despite some ATL weakening is far stronger in manpower and resources and European allies than Portugal, or more likely Portugal remains Catholic and then if England goes Protestant that alliance is suddenly reversed, which means any English bases overseas that were conveniently near Portuguese bases suddenly will be caught in fighting between the former allies. In that case I suppose Portugal has little choice but to ally with Spain. OTL the dynasties merged for a while; I figure in the ATL the intermarriages that led to that would be diverted away, but a political alliance between two monarchs seems just about inevitable for Portugal then. The remaining possibility is that England does not go Protestant, but while it is hardly inevitable England must do so, it does strike me as quite likely due to a bunch of reasons that prevailed OTL and will more or less exist in the ATL strongly too. The last straw of Henry VIII being frustrated in not getting a male heir from Catherine could easily be butterflied away, indeed it might not be Henry at all, but all the reasons his court lined up pretty strongly behind his rebellion versus Rome would still be considerations. I can imagine Portugal remaining Catholic and strongly so, but an alliance with an England that remains technically Catholic with its monarch professing that faith but tolerating a lot of Protestant dissenters; that seems the easiest path to continuing an alliance between them amidst the Reformation--this would surely moderate or even completely block any English support for Protestant uprisings on the Continent, so the Netherlands might fail to secure their independence from the Hapsburg system. And so on...
The most ASB part of this whole fantasy is the part about getting significant help from South American Native allies, first of all their surviving at all as dense and high population civilizations, which is key to the whole thing, and then the Portuguese having such a different approach to domination of them versus the usual European pattern of being pretty overbearing. Frankly the first part is the biggest weirdness to swallow, given how total the collapse of Amazonian civilizations was OTL and how rapid, and I am musing about a frankly ASB scenario in which the Amazonian peoples have stumbled upon some herb or fruit or what have you that is a wonder medicine of sorts--in high doses, it revs up the immune system a lot. This is inherently risky in itself, much of the harm diseases generally do to people is actually side effects of the immune system attempting to defeat the infection; taking the Amazonian remedy potion in high doses might be fatal in itself, but with plagues ripping through the population they might attempt desperate measures and find they work, enough to preserve say a quarter of the population, with half the rest dying from the diseases directly and the other half dying or being severely disabled by the remedy. In addition to merely preserving the Amazonian peoples in being as civilizations capable of sustaining their elaborated agricultural system, such a wonder remedy might itself become a hugely important trade good. Say it is difficult to transplant its source plant to other tropical rainforest climes, it happens eventually but it takes a while, and of course flatly impossible to grow it (outside of greenhouses that are also hard to get right for a long time) outside such climates, so Brazil remains the sole source. And indeed the Native people keep important secrets and use the leverage offering to trade in it to negotiate their somewhat equal status versus their Portuguese partners and gradually they Europeanize enough that they are indeed in an alliance and eventually, long before OTL, South American states with a Native majority population and continuity with pre-contact Native polities are nations with diplomatic presence in Europe--as Catholic nations they have their own bishops, archbishops, eventually Cardinals, for instance, and there might possibly be a Pope from among them perhaps long before the 21st century (this is unlikely to be sure, but not impossible).
And plenty of other knock-ons--the Amazonian converts having a fair stock of the more or less processed material on hand, the joint Portuguese-Amazonian expedition to Peru travels with a lot of it anticipating exposure of its members to smallpox and other diseases running wild in the region, and can use the offer of possibly curing people who are desperate with little to lose to win over more allies and secure a Catholicized new Andean dynasty whose alliance is secured in part by preferential access to the medicine. The medicine as a trade good would be tremendously valuable of course--not as good as if it could be taken without risk to be sure, but European history of this era is rife with plagues and various kings and rich people would be vying to secure some of it for themselves and market more of it to the general population--actually I suspect kings will tend to delegate mass administration of the stuff when there is enough to the Church; conceivably doing this early enough could butterfly or anyway divert the course of the Reformation on the Continent. If favored armies first of all have special access, that will be a force multiplier as their troops will be somewhat less decimated by diseases and perhaps recover better from wounds. And in general the more of it is in use, the stronger population growth will be. Amazonia itself I assume is devastated initially but between the population collapse being limited initially, the survivors being able to retreat to the most choice land and becoming comprehensively effectively as immune as European populations or more so, and a certain small influx of Portuguese and other European settlers mostly intermarrying with Native people, population there will start from high levels versus OTL and rise back toward and probably well beyond pre-contact levels fairly quickly. Broadly speaking the same will be true of any Peruvian peoples that come into the system. As availability of the medicine increases more and more population growth versus OTL can be expected in Europe while European stations in tropical disease-prone regions would be more viable; a certain amount of European attention would therefore be diverted to Africa, along of course with lots of powers trying to horn in on the Amazonian source and major crop zone. Of course the stuff would be sought after by non-European powers too; the fact that the Portuguese and eventually rivals of them can offer it in trade at Chinese ports might be a big game changer in Sino-European relations, and generally, there would be more and earlier contact between European traders and other powers, at a time when European tech was not so much advanced versus the general level of Old World gunpowder empires, so these would stand a better chance of incorporating key techniques and be more robust against later European encroachments.
This is an ASB TL of course!
