Success at last! Tonight the MPG-200 successfully controlled the JX-3P for the first time! The current circuit uses the same japanese transistors as the PG-200, but I am still not sure that is necessary. I have tweaked the delays between the address and data bytes (the timing in the PG-200 is about 70uS between the last low of the address byte and the first low of the data byte) and between two blocks of data (PG-200: at least 17.5mS). The current delays for the MPG-200 are 91uS and 20.4mS respectively.
Now I have to get this to work with a more custom delay (or at least drop bytes instead of queueing them up, currently I get a 'delay'-effect of sorts instead of stepping. I also need to figure out how to generate bit masks for the switches.