Ben, I have been thinking the exact same thing for when I finish my bike. Check out renewable energy shops, there are generators made to directly charge DC batteries at different voltages for when solar/wind powered systems aren't putting in enough juice. I can't remember if you are running 36 or 48V but there are definitely options out there... here is a 48V 60A
Australian option I was looking at. It would cost more than my entire build but i'm sure there are other (cheaper) options too. It would make for the perfect range extended EV without having to carry the extra weight when its not needed.