I don't currently have plans on adding any XML-RPC or JSON-RPC stuff, granted JSON-RPC is very easy to do on your own. This is mostly because I don't do much XML-RPC stuff, however based on user feedback and how I could improve on existing solutions, I would consider it.
Have you used any other XML-RPC libraries out there? If so, do you have any feedback on what they are lacking, whether it be usability or portability, or something like that?