I recently finished modifying my main axe, a Roland G-202 guitar synth controller, installing a set of RMC bridge piezoelectric pickups in addition to the original GK-1 hexaphonic pickup electronics. This allows simultaneously controlling both an Axon-100MKII (digital, MIDI) and a Roland GR-300 (analogue) guitar synthesisers. This was to avoid resorting to trying to build a 24-pin splitter and BC-13 (which runs the GK-1 pickups at +/-7V rather than the original +/-15V.
I run the RMC piezo pickups to an 8 pin connector on the jack plate which then connects to an RMC PolyDrive II external preamp. I replaced the original (impossible to find) 24 pin connector with a DB-25 female version, and using the original male connector, built a 24 pin cable to connection to the GR-300. So I didn't need to modify the GR-300. The G-202 has already been substantially modified (Seymour Duncan pickups, LSR machine heads), so there wasn't a need to keep it stock.
There are some photos of the job if people are interested.