I suppose you're right. Some of the arrangements have some pretty complex layers of synth pads in them that simpy don't gel with what he's playing on the piano, unless a "Brain2MIDI" interface exists that I'm not aware of ;)
From looking at the Live 2003 DVD (the Hollywood Bowl part in the diary section), there's a shot of the GT20 with a Memorymoog hooked up to it, so I'm pretty sure he's driving some synth or other when playing live, and use a backing track on some of the other songs they play. Either way, it works very well :)