> I don't know yet how Windows do decode bytes filenames
> (especially how it handles undecodable bytes), 
> I suppose that it uses MultiByteToWideChar using cp=CP_ACP and flags=0.
It's likely, yes.  But you don't need a new codec function for this.
What about something like .decode('mbcs', errors='windows')?

I still don't see the advantage of codecs.code_page_encode().