Thanks for working on this. I'm down for joining betas (I was one of the original PaperPile beta testers) if you are looking for feedback on this feature. Otherwise I'll wait for the official release.
Google should be somewhat embarrassed that you can't restrict your Drive search to one folder. I think Drive used to be able to do that. Gmail still can, so this means that Google -- the search company -- has provided better search functionality in their email app than in their cloud storage. Go figure.