Hi Craig,
I don't know too much about integrating Joomla, or all of the xml-rpc processes that occur in moodle, but looking at your website, I wonder if you actually need them to be integrated?
Do you want people to be logged in across multiple domains?
Perhaps you could just point moodle at the user table for your joomla database.
Don't know if this helped, but it looks like your question has been unanswered for quite some time.
Good luck!