None of the other options really have much use in this case, since they're mostly about how to combine multiple environments rather than excluding implicit ones.

Setting PYTHONHOME to the current directory certainly seems to be enough AFAICT, at least for Windows, as that bypasses the most problematic registry lookup. Having an "applocal" option means we can go further and ignore all registry keys and environment variables.

I haven't looked into how to duplicate the behaviour on other OSs, but Modules/getpath.c looks similar enough to PC/getpathp.c that I guess it'll be similar. Adding the MacOS folks in case they have suggestions/concerns.