Marsupial,
Could you post any websites where it gives info on interfacing the RM-X4S with the PC as this is what I'm looking to do. You say about interfacing it with the joystick port, but what about interfacing it with the serial port, as this would seem to make more sense (?).
Having had a quick google, I found this website, which interfaces via a serial port, but to an Empeg car stereo rather than a PC.
Could be of help though - might be worth emailing him to see if he could help us with our quest. (I've no experience of PIC programming though).
http://www.armory.com/~charlie/empeg1/index.html