Here is a patch for discussion that allows non-iterable choices with or without metavar.  It refactors as suggested.  However, the patch does not yet have tests.

> Does the test suite already have a testcase already for non-iterable choices + metavar?

No, not that I saw.  Also, to answer a previous question, the three places in which the choices string is used are: in the usage string (separator=','), in the help string when expanding "%(choices)s" (separator=', '), and in the error message text (separator=', ' with repr() instead of str()).