Good :)
Once I was able to achieve something similar to what you are asking.
There was a mod around for Cadaver's GoatTracker that implied a .dll file, a real C64 and a DIY cable. Parallel port of the PC <=> user port C64, I'm not sure, it was quite some time ago, but I do remember that worked only in a DOS environment to me so I had to use a Win98 PC.
I don't remember the protocol involved but the speed of the communication was similar or slightly faster to that of MIDI, you basically had to restrain yourself to 1X SID.
Let us know if you find something, it's interesting.