How about allowing for the +12 volts to come down the Serial cable into the DB9 connector [with maybe a simple on/off jumper to allow this option to be disabled on the PCB], as an easy way of getting the necessary voltage into the PCB to run it.

Recall the Empeg has +12 volts already available on the DTR line (pin 4) of the external Serial port on the sled, so this would make a "single cable" solution, with no cutting of wires or splicing into harnesses to be done in most cases - juts plug the DB9 connector to the Empeg's sled and away you go.