I've seen posts about mounting a small portable 120V generator on an ebike to power the charger and extend the range. Most gens are too heavy and wide.
Here is one that weighs 30lbs and may be narrow enough for frame mounting.
However, it will cost around $400 shipped. http://www.electricgeneratorsdirect....Proof_2009.pdf
It is great concept to use bike mounted generator, It will help from regularly charging the batteries and also it can easily lift heavy weight. The main benefit of using this generator is, it will not take much space and can be carried easily.