via this auction
Full details on the update here.
So I got hold of the service manual and the datasheet for the 6303 processor used in the assigner board, and set about dis-assembling the firmware, in the hope of finding the 'bug' that stopped SysEx sound edits working.
In the process I learned a lot about how the JX works, how the PG800 works, and how NOT to write firmware for a synthesizer. The Roland software engineers must have either been under real pressure to get the job finished, or they simply got the office cleaner to write it, because it is a poorly structured, badly written mess.
When I found the code for SysEx reception there was only a slight hint that they had planned to implement editing via midi, no 'broken' code for it, but with a little thought, I found a way to hack it in there myself..."