> I also think that nargs=## could maybe be special-cased to just ignore 
> the A/O designation completely and only check there are enough, but I 
> haven't tried this out. Does this seem like a viable approach? Would a 
> patch that does that, subject to some flag, be of interest?

I can't leave well enough alone, so, with the following additional patch:

-    def _match_argument(self, action, arg_strings_pattern):
+    def _match_argument(self, action, arg_strings_pattern, arg_strings, start_index):
+        import numbers
+        nargs = action.nargs if action.nargs is not None else 1
+        if isinstance(nargs, numbers.Number) and len(arg_strings_pattern) >= nargs:
+            return nargs
+
         # match the pattern for this action to the arg strings
         nargs_pattern = self._get_nargs_pattern(action)
         match = _re.match(nargs_pattern, arg_strings_pattern)
         ...

Then I get this:

  >>> import argparse
  >>> parser = argparse.ArgumentParser(prog='a2x')
  >>> parser.add_argument('--asciidoc-opts',
  ...     action='store', dest='asciidoc_opts', default='',
  ...     metavar='ASCIIDOC_OPTS', help='asciidoc options')
  >>> parser.parse_args(['--asciidoc-opts', '--safe'])
  Namespace(asciidoc_opts='--safe')

Comments on this approach?

(Again, I haven't run tests, it'd need to be controlled by a flag per your desire to not change existing behavior, etc.)