Ideally, I'd like to find one that I could make run off of 12 volts. Then a simple cigarrete lighter adapter could power both the empeg and the FM transmitter.

Hey hey, here's a thought for a project: FM transmitter guts, running off of 12 volts, small enough to fit inside the casing of the empeg. If desired, on/off switch and/or frequency selector peeking out of the security slot.