The Inverter series generators from Honda use a DC generator but I am not sure what voltage they output to the inverter.
Your treadmill motor idea sounds good, but I am not sure how you can properly size it. Does input power as a motor = input power as a generator? I really don't know.
You are also going to need to throttle the engine to avoid over charging.
Pick up a copy of "The Zero Carbon Car" from your local library:
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The guy in there made a bio-diesel series-hybrid Miata and managed to do exactly what you are attempting, but I forget how....