I see, you're considering it as more if a charger than a direct power source, sorry I missed that the first time. It's an interesting concept. If we think of it in net power terms, lets say my eBike is running at 1.5C off my 20AH pack on average, that's 30 amps. The maximum charge rate allowable for the pack is 5 amps, at 43.7 volts, or 218 watts. The genset is 350 watts and with conversion inefficiencies it's no doubt capable of that. So we are adding 5 amps to the system which you can either choose to think of as 5 amps charging and 30 amps discharging, or more simply 25 amp discharging (the gas engine is really just helping to power the electrical one). This means I'm pulling 16.6% less current from the battery (5 / 30) so I'll probably go 16.6% further on a charge if we ignore the weight of the engine, gas and oil. That's 60 miles instead of 52 miles.
That's still not a compelling case for me personally. Especially not to haul around another 18 lbs. I'd rather attach another 20AH pack at 15 lbs and extend my range to 104 miles, ignoring the pack weight again.
Some would argue an eBike is already a hybrid system, what I think you have in mind is a trybrid (tribrid?) system
Still, if you pull of the engineering challenges and get a prototype ready, I'll be a guinea pig
