So it should end up in Lib/importlib/_bootstrap.py since it is directly part of how import works. The original code as found in Python 3.2 for PyWin_FindRegisteredModule() is referenced in Python/import.c and is implemented in PC/import_nt.c. You will want to create a new sys.meta_path importer (if I'm reading the code from Python/import.c correctly in terms of the call to PyWin_FindRegisteredModule() happening before sys.path is traversed). You can refactor FileFinder if you need to in order to reuse its code.

In terms of working with importlib, you will probably need the winreg module, so inject that (see _setup() on how to do that). Also remember that while this is Python code, you will need to rebuild to trigger the regeneration of importlib.h. Otherwise (I hope) it isn't too complicated or hard to understand.