I have wanted this for some time, and someone asked on mailman-users, so I hacked something up; being better at CVS than I have been, I made a patch and posted it.
Here's what I posted to mailman-users:
Patch #100953 (on sourceforge) allows list-specific files; you put them in ~mailman/lists/<listname>/ , and they're used in preference to the ones in ~mailman/templates for that list.
It was a bit dodgy how I had to do it; the list object wasn't previously passed to Utils.maketext(), so I had to find all the callers of maketext and add the list object to the call params. Mostly, the change is in Utils.py, but there's a slight touch to a lot of other files.
I'll submit this over on mailman-developers and see if anyone has any comments, but it seems to work. Of course it's only valid against the current CVS snapshot as of right now (Barry's been changing a lot tonight preparing for beta5).