I very much doubt that anyone would accept that a potential heir to the French throne sit on that of Spain.
This is asking for a "War of Succession, Part Two: This time we will stay with Menorca!" in the eyes of the British. And I wouldn't be surprised if the French proclaimed a Republic after the fall of the Second Empire to avoid that risk.
(This assuming that Spain does not give in to pressure to place someone who cannot be used as an excuse to create a Franco-Spanish union).
I'd worry more about French intervention. Before Leopold was offered the throne, Napoleon III was paranoid about Montpensier getting it.