The only difference between xmlrpclib.py from trunk and 2.5.1 is in the
Marshaller class. Unrelated, as far as I can see.

Note that it seems that the intent of the original code was to support
this x509-dict all along:

$ grep -n x509 xmlrpclib.py.trunk
1224:    # Host may be a string, or a (host, x509-dict) tuple; if a string,
1228:    # @param host Host descriptor (URL or (URL, x509 info) tuple).
1230:    #     x509 info).  The header and x509 fields may be None.
1234:        x509 = {}
1236:            host, x509 = host
1251:        return host, extra_headers, x509
1262:        host, extra_headers, x509 = self.get_host_info(host)
1282:        host, extra_headers, x509 = self.get_host_info(host)
1362:        # host may be a string, or a (host, x509-dict) tuple
1364:        host, extra_headers, x509 = self.get_host_info(host)
1372:            return HTTPS(host, None, **(x509 or {}))

Basically just the ServerProxy constructor doesn't support it. One would
have to create a new class with a new constructor just because of it.
That's why I opened this ticket.