Well, every Grubee motor I have installed including the new 2010's have a White wire coming out for raw ~7VAC you can draw 3 or 4 watts off of.
You can buy them for $8
Bicyle Motor Sparker Wire Loop Set Magneto Parts - Motorized Bicyle Sparker Wire Loop Set Magneto Parts - Bicycle Engine Sparker Wire Loop Set Magneto Parts - Motorized Bike Parts
I mean come on, it's just a copper wire coil ;-}
(per you PM) you can step-up the transformer AC up to 12VAC but you would need a battery and DC to store it, and you'd get a trickle charge at best converting it to DC to store it.
I have no first hand knowledge of a self sustaining electrical system for an MB yet, but like you FreeAtLast, an electronics degree and a couple of decades of experience and know how I am going to start.
I am going to start with one or two of these 12V 6W 'tire rub' generators for $17
Lights - Generator 12V | BikeWorldUSA | Bicycle Parts and Bike Accessories
Then mount a 12V battery like this for $16 and full wave rectify the gens in parallel.
Sealed Lead Acid Battery 12V 4.5AH (Upgrade to 5AH) for UPS & Emergency Lights (S)
That will give you some serious Watts.
If you spend more time riding during the day with no power draw it should be able to power some serious lighting you can actually use for an hour or two at a time.
Again, I haven't tried this yet and there are drawbacks to tire-rub generators like sidewall wear, but it sure beats messing with the inside of the motor, your CDI pulse comes from there, mess that up and just like screwing up your cars timing, it won't run.
Just my 2¢ worth.