Personally, I don't like overloading the bit like that, but I'll limit myself to pointing out the problem. Personally, I don't see how anyone would end up depending on this behavior, and I don't have any inclination towards following standards when they do silly things. It would nicely bypass the question of what to do with the set-id bits. I've tested that this does the right thing for root and users, but haven't properly tested it for how it acts with piecemeal capabilities. This needs a really good eye-balling to make sure the logic is right.
Testing is irrelevant.
Very poorly tested. Completely untested, but trivial. If force is set do it anyway.