It really needs to be fed into a battery, with proper regulation to feed the Pi.
Also, I've found, at least with wind-up torches and the like, that they're remarkably inefficient - most of the work put in goes into the gearing, not the electrical output.
I have a cheap brand wind up radio - simple plastic gears and a standard toy motor. Tops up two small (half-AA size?) rechargeables of some sort, so we're talking about 2-3 volts.
Yes it'll need over-voltage protection too, and I suppose some proper charge circuit to prevent the batteries getting overcharged.
Also it puts out a huge amount of RF interference - so ironically the radio is unlistenable while you're winding it. By the way this even interferes with my digital (DAB) radio so might upset other things in the area like wireless networks?
So I suppose it could be done with a decent generator, or a bike and 6V dynamo sounds good (or large solar panel), but we'd need a big battery...