"Dragging" a generator behind you (hopefully on a small trailer of some sort) sounds like a bad idea on a bike like Enertia (or any bike for that matter) for more reasons than one.
1) As Protomech said, you can't charge and discharge at the same time. It may be possible with mods, to run off the generator, may be with a switch of some sort, when you run out of juice in the batteries, flip a switch to charge the battery and run off the gen...
2) Even if you did something like what I mentioned above, the added drag from (at least) two tires for the "Trailer" would add a lot more inefficiency. It would also make for a less aerodynamic profile, hence more inefficiency.
3) Add to that the additional weight of the Gen and fuel; about 50lbs more weight plus the weight of the "trailer". That's more than 25% my weight.
4) Safety and handling. Breaking would be a concern. Coming to a safe stop fast, is probably more important than getting to speed in a hurry. I've never seen a bike tugging a trailer, and there's probably a good reason for that.
Also I'm assuming you'd use a trailer, unless you strap the gen to your back which doesn't sound like a very bright idea either.
Unless someone came up with a tightly coupled hybrid system, I don't see this working very well...
Even then, the added complexity, weight and price for performance may make it unfeasible...
As Brammofan put it, the main reason most of us are getting electric bikes is to get away from gas.
SamM you said unless you got a 60 mile range, it may not work for you. well guess what, there's light at the end of the tunnel. Enter Enertia Plus and Empulse. Both available (hopefully by mid next year) soon with more than a 60 mile range.