After writing the above, I discovered that IDLE *itself* does not use any particular encoding for config files.  Its configparser.Configparser subclasses load function calls .read(filename), which in turn calls open(filename, encoding=None), which calls locale.getpreferredencoding().  For me, this is cp1252.  (Changing the console codepage with chcp or the process locale with setlocale don't change this.  Something else might.)  So this issue currently has nothing to do with iomenu.encoding.  Rather it is a proposal that IDLE explicitly pass a locale, in particular 'utf-8'.  This has the same issues given above.