You mean like this?
I think I did a post on my gas/electric here a bit ago.
The difficulty is getting current to the field circuit of the alternator to get it to make power.
Don't know about hub motors, I used a 24v 450w motor and chain drive. The engine pulley is NOT the big one pictured here- one to one ratio now.
Here is what it looks like now (all stealthed out)- max speed 20 mph, max voltage out of the alternator is limited to about 35v by the shear load of driving the alternator by the HF79.
Uses more fuel than a normal bike because of inefficiency, but works for me and will be legal when the governor gets around to signing the new E-bike law. Note- the bike is real heavy at 113# but the rider is light.
Handles super with the 3 inch stretch frame.