> Is the following reasonable: The IDLE shell console has a locale of
> "non-BMP utf8"?

[BMP utf8]
That's indeed the approach that Andrew and I were discussing. 
Unfortunately, there is no codec for it yet. We were discussing
to add a "utf8bom" encoding to Python. This is a medium-sized
project, though (and again out of scope for this issue).

> If so, would it be reasonable to add a menu item to switch locales
> for the shell? This amounts to adding some extra code to
> OutputWindow's write() to raise encoding errors if the string
> contains unsupported characters, and possibly replacing characters to
> work around Tcl/Tk's non-BMP limitation.

Please open a separate issue for this.