As it turns out, though terms could not be edited without affecting all instances, they could be deleted without deleting all instances.
So, it looks as though the answer is this:
- you cannot separately edit a term that’s in multiple taxonomies, but
- you can remove that term from a taxonomy without removing it from the
other taxonomies it appears in.
For related ideas, see http://www.wpquestions.com/question/show/id/3005.