Wow, what an awesome bit of gear. I'd love to hear some more info about it DJ. Is it based on a factory bike or prototype? or just what the guy would have liked his works bike to be. The workmanship on it looks unbelievable, some serious hours in it i'd imagine.
Tell him not to scratch it too much until VMX mag have done a feature on it.
I reckon rules only come into it if you're racing for trophies. Whether it is legal at a champonship though is a legitimate question I suppose.
In a similar vein, I was talking to some guys at the track the other day about hand/thumb operated rear brakes. There was a version on New Inventors a little while back, and Mick Doohan famously started using one after his leg was mangled. Just wondered whether if you rigged something up yourself, there'd be any sort of problem, given that it is in no way standard.