I can't really believe something like this doesn't exist, or has at least been proposed already, but my half-assed searching turned up nothing.
It's a battery charger, at the end of a rectifier, at the end of an alternator, at the end of a gearbox. Why hasn't anybody every done that?!
odd part is this. Rather than hold one end and turn the other, as the hip and trendy campers do with their wind up radio/flashlights, why not have one end spin free?
The simplest form would have a loop extending circumfrentially from the shaft of the gearbox. You clip in something with a bit of weight and start twirling it while you walk.
As it starts up, there's only a small resistor and a transistor in the circuit. Once it spins up, the device can ramp up the load, then cut back when RPM starts to drop.
As there's already some moderately sophisticated power regulation going on (I hope; you do love your batteries don't you?) you might as well add a variable intensity LED flashlight.
And since you're going to have LEDs, why not a hemisphereical camp lantern as well as a spotlight? And then you might as well have it flash SOS, overdiving the LEDs, for emergencies.
And since we're preparing for emergencies, how about a bit of NiCrome wire for emergency firestarting? And and AM radio? A 406mHz PLB?
And in case that isn't enough, build it into the handle of a folding shovel. Then you could twirl the head of the shovel to charge it, or plant it on a hill for signaling.
Ok, I'm pretty done. Realistically, a flashlight and battery charger doesn't seem impractical. Even the heating element, and maybe the AM radio, could be done as a weekend garage project.