The Seiko Watch Co put a quartz movement(?) in their watches which, was charged by a centre pivoted weight with tiny two coils on it. The generated charge was stored in a capacitor I think. I have three of these watches that do not work anymore (two from my late father), and the repair costs are steep. It is a pity, as for 10 years it ran well and kept good time. There are also torches that can be charged by shaking a weight through a coil. The circuitry gets smaller and smaller, as do the led's that have replaced the globes... While on pickups, the microphones that you can clip on are also small, but you either use phantom power or carry a small battery case on your body. If a small system could be developed onto your strumming arm charging into a capacitor, maybe that could provide the "green" phantom power instead?