The PoD would have to be before Hindenburg's death. The old Kaiser can't just appear out of the woodwork and ask for his throne back. However, if a monarchist movement gets underway while Hindenburg is still alive, it is conceivable that the President could perhaps decide that he wanted Wilhelm to be his successor, subject to a referendum. It would be difficult, though, because Hindenburg, Hugenberg and the like may have already given Hitler the chancellorship in order to "contain" him. If they have, then it becomes all the more difficult to tell Hitler that the Weimar establishment was okay with him having a certain amount of power (the chancellorship), but not quite that much (President).