Best bet for me is either D.Pedro II choosing to resist the coup(but his personality makes that improbable at best), or D.Isabel heeding Gaspar Silveira Martins' proposal to get her heir to Brazil during the Navy/Federalist Revolt(but that would need better military and political leadership on part of the rebels to win over the government forces).
After that, things get harder. Unrepentant monarchists were purged(bloodlessly, but purged nevertheless) from Brazilian political life. By the late 1900s, they were beginning to die, which means the Imperial Family doesn't have a political base, and would depend on the ones that would bring them back to power(the Imperial Family fully knows that, which is why, IMO, they insist on returning 'only with the assent of the majority of the population' - to return otherwise is to become puppets and rubber stamps for the ones who put them back in the throne, at least in the beginning).
Still, I can see two (long shot, to say the least) possibilities. The first one is Vargas bringing them back. This would have to be after 1940(which is when Vargas created the Imperial Museum in Petrópolis, as part of the efforts to paint himself as the 'D.Pedro II of the 20th century'). So... say that in response to the Manifest of the Mineiros(so, 1943), Vargas pledges to bring the Constitutional Monarchy back 'after the end of hostilities', instead of going full populist in order to consolidate a political base of support. The Army will be pissed, and this will have to be dealt with. OTOH, they can gain support among those sectors that are close to the Catholic Church(not enough, so Vargas will have to do really well in political maneuvering, even better than his usual). The Imperial Family will be wary, but I think Vargas can play the Petrópolis and Vassouras branches against each other here(note, I agree with holycookie that the Petrópolis Branch is the legitimate one; that wouldn't matter to Vargas, though). By, say, 1946, a new Emperor sits on the throne.
Second scenario is even more unlikely: in the period between the 1929 Crash and the 1930 Revolution, one of the Princes starts agitating for the end of the Republic, saying the current events prove the Republic has failed, and starts to form a political movement and army(because there is no way this wouldn't end in civil war) to put himself on the throne. This scenario is, frankly, almost ASB, and the 'Prince' in question would have to be for all practical effects a created character, because I can't see one of D.Isabel's sons or nephews even considering this(the only one I could see doing that is D.Pedro Augusto, but he was in a mental asylum, and would die in 1934... hmm, perhaps that could be a third possibility), but it could be a good story.
