This was a concept I was working on for a trailered generator rig, but never did any experimenting:
120V AC generator with voltage regulator to hold close to 120V
Transformer to step down to a voltage just above trickle charge voltage at the battery pack (must consider voltage losses in rectifier and filter)
Bridge rectifier to convert to pulse DC
Coil-capacitor filter to smooth DC
The output voltage DC would be held at a point that would charge the batteries above a trickle charge level, but not enough to cook the batteries during long rides. While under load, the voltage drop on the DC side would cause the AC side to regulate and add power to the gas motor. If the generator could not handle the instantaneous load (for example when first starting out), the batteries would kick in to keep the DC voltage level at their charge level.
I was looking at the Honda 1000W model because it was a nice package and seems reliable. But I don't think you need anywhere near 1000W, at least for my 600W brushed hubmotor. If the batteries provide amps during peak load, and the rest of the time they are in charging mode, I bet you could use a very small gas motor to power the generator, maybe more in the 500W range. My guess is that the generator would need a little bit more than enough wattage equal to cover power use during top speed runs on a slight incline.
Its all theory though, I don't really know how hybrids manage to regulate battery charging and motor control. There may be a better way using a DC generator.
But this is a lot more cost effective:
http://www.staton-inc.com/Details.asp?ProductID=2729
Yes, I know. The dark side.