I'd suggest not worrying about the default metavar in the _expand_help() method.  The formatted choice string created in that method is only used if the help line includes a '%(choices)s' string.  The programmer can easily omit that if he isn't happy with the expanded list.

TestHelpVariableExpansion is the only test in test_argparse.py that uses it.

        Sig('--foo', choices=['a', 'b', 'c'],
            help='foo %(prog)s %(default)s %(choices)s')