It appears that the failures in msg248192 and msg248365 are issues with non-latin1 chars in general, not with astral chars in particular.  Anyone who wants filenames with astral chars should be using a utf-8 locale.

This issue is about Idle working around the tk BMP limit where it can so as to not prevent editing and running files with system-legal names. According to msg248188, the patch works.  So I am closing the issue as fixed.