One potential problem I see is that if the generator is charging the car and you hit the brakes and it goes into regen mode it could over-amp the battery. Remember that the car isn't designed to drive and charge at the same time. But an easy solution would be to put a contactor on the charging line that opens whenever your brake lights are activated. And just don't use B mode. That way you'll know that you're not over-amping the charging of the battery.
Another thing would be the wiring. It would look kind of funny having a chord that goes along the top of the Leaf to the charging port in the front.
But yes, I agree that a generator system would be useful, even if you had to drop the battery to route the charging wires from the rear. (Then you could park either way at charging stations.
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I think my idea to convert an electric motor into a high voltage 12kW alternator would be cheaper and produce twice the power that you could get from a generator. The charging system in 2013 and later Leafs only supports 6.6kW charging (in 2011 and 2012 models it's half that.) If you get 100 miles out of 50 miles an hour (two hours) then your using about 12kW of juice. A 7kW generator plugged in to the Leafs charging system via a 6.6kW charging cord will give you about double the range, 200 miles, before you have to sit and wait 4 hours for the battery to charge up again. A 12kW system or higher would be closer to being able to drive all day for hundreds and hundreds of miles without having to stop and charge up every so often.
Also you would want to be careful with the generator you chose. Generators tend to be hard on electronics. About the only electronics trustworthy generator that can produce 7kW is the Honda EU7000. Actually, I'm thinking of getting the smaller Honda EU2000 for level 1 charging and keeping it in the Leaf for emergencies.
But on that same note, a motor converted to an alternator would produce a DC voltage that could be more easily smoothed out.