Too bad auto alternator efficiency is so low, they are pretty controllable. I wonder how efficient a 3 phase generator re-configured to make DC, controlled with an alternator regulator(only excited enough to produce 12, 24, or 48VDC) would be as a battery charger?
Back to system efficiency, how will short cycling your batteries effect their lifespan? An efficient generator regime might not seem so efficient if it shortens your battery life by a year... Now if you used a hot water type heating system, and had a well insulated storage tank, and could find a source for assorted wattage heating elements... I am sure a controller could be programmed to switch them on/off in particular combinations to keep a constant load on the generator for a full battery charge cycle. Then the heat energy would at least set for a while in storage to perhaps be used to maintain house or domestic hot water temperature. For that matter the waste heat from the cooling system could go this route as well. This might keep a gas fired waterheater from cycling as often. You are burning the fuel, might as well use as many of those BTU as possible.
This would seem like a good match for a solar electric and solar hot water system. When the sun is available, the electric and heat energy is provided by these systems. When it isn't available, you burn some of that liquid sunshine to put energy into these systems