The TV is USB bus-powered (or it is now, after I took it to bits and attacked it with a hot soldering iron), so I now just plug a USB into my MIDI breakout board to power both this and the Pi, the Pi is powered by 5V on the GPIO header. Cool! And it was only £15.
Also - this Piana is running 8 note polyphonic, with the filter in place. I need to benchmark the new modulation code, and double-check my overclock settings (I think I'm at 900) but it looks like I've saved enough cycles for 8 note polyphony to be reliable.