[samatman]

I've done enough bike wrenching to have raised an eyebrow on that.

What you need is two gears on the flywheel, rather than one, a gear rather than a friction wheel on the generator, and some way of tensioning the second chain which you run between them. This isn't the kind of harder to build that should stand in the way, although I will grant that the single gear was already on the wheel and changing that does involve, well, changing that.

This gets ~10% efficiency back, which for a generator is huge. It's probably the only efficiency gain left, other than a gear cassette to optimize power to a target voltage.

[SamBam]

I'm wondering if the energy loss through the tensioning and/or internal gears is at all significant.

Instead, would it make sense to have a heavy flywheel, a fixie gear, and a way to slowly ramp up the power draw, so the power draw itself is the "gear" that allows you to go from a hard-to-start heavy wheel to a smoothly-running wheel?