Having a modified utf-8 codec will be useful.  That said, it is an error
for Tcl/Tk to expose modified utf-8 externally, and that was fixed at
some point in Tk8.5.  Since Tk is no longer sending 0xC080 for the %A
char, switching codecs in _tkinter.c won't accomplish anything.

This fix was to correct a long-standing problem in IDLE using Tk8.4,
which is most easily solved by catching the leaked invalid null in
_tkinter.c.

It seems to me that, once you switch to modified utf-8 and allow the
embedded nulls, you have to make sure everything you are doing uses
the modified utf-8 encoding/decoding.