client_address is not interresting, but I prefer the handler object (SimpleXMLRPCRequestHandler instance). The handler contains more information:
 * handler.address_string() gives the FQDN
 * handler.connection.getpeername() gives the client address
 * handler.connection.getsockanem() gives the server address, useful is the server is listening to multiple addresses
 * etc.

If the server uses SSL, it would be possible to get more information about SSL from handler(.connection).

Your patch breaks the API for all (service) callbacks. Add an option to register_function() would allow a smoother transition (don't break API): you don't have to inherit from a new class. Eg. server.register_function(whoami, with_handler=True).