How ever that is implemented, it must be implemented system-wide, or you will end up in chaos.

We already live in chaos. Python implemented it python-wide, and arguably resulted in less chaos than Perl.

Another option is to pair the string of bytes with the best guestimate of its encoding within some sort of path object, and then be able to flatten it back to the same bytes it came from, and also answer questions about what it would look like in unicode and how confident we are about it's encoding. I'm proposing wrapping the paths in an object anyway, so maybe that's what I'd do. I need them to stringify back to bytes in order to interoperate with the rest of Perl, anyway. Python gets the advantage of the whole language ecosystem respecting the remapped invalid characters, so they can pass filenames around as plain strings.