after a bit of you tube trawling, I found this
http://www.youtube.com/watch?v=T7D44i8cJYo
it uses a small RC Zenoah G260 motor. very nice.
no information on the alternator either. wattage output or voltage.
there no reason why you couldn't add a chain to this an let you bike recharge through regenerative braking function?
I doubt if this system exists
would somebody like to build one as a lightweight emergency charger?