y'know i'm not trying to stomp on your dreams, right sam? just giving you the cold hard truth...
BUT, i was thinking, if you built a mountain bike, or a trail bike or something, you might be able to register it as an off-highway vehicle (OHV) and ride it wherever people ride atv's and motorcycles. i didn't see anything banning bikes, except for one thing banning pocketbikes.
it also looks like you could legally register the bike for on-road use (see the "real deal on california laws" thread i posted) and then just pay a fee for off-road use.
i know this ain't the best solution, but at least you could have some "weekend-fun..."
here's a couple links to check out:
California State ATV Information
then check this out and scroll down to "Registration requirements for a specially constructed OHV:" How To Register an Off-Highway Vehicle