Looking at the current roundup/cgi/client.py I see:

           elif self.env.get('HTTP_AUTHORIZATION', ''):
                # try handling Basic Auth ourselves
                auth = self.env['HTTP_AUTHORIZATION']
                scheme, challenge = auth.split(' ', 1)
                if scheme.lower() == 'basic':

which to me looks like it has implemented what was requested.

Probably as part of issue1067690 which is closed.