This made me think of a piezoelectric generator. It generates power when mechanical stress is applied. Amount of power generated is dependent on the amount of mechanical stress applied. It has an internal buffer and it's output is capped at say 20 RF/t (or scaled number of power of your choice). Punching it creates 40 RF so you can build up the internal buffer by continuously punching it for a bit. A nice little early game engine (somewhat akin to the Forestry clockwork engine). I think it should not respond to an autonomous activator for balance reasons. One generator isn't much power but with enough piezoelectric generators and enough autonomous activators you could have quite a bit of free energy.
It could also generate power by jumping on it. Think a floor made of piezoelectric generators at the bottom of a mob drop trap.
Walking on it would also generate power. Line up a few and sprint over it for more power. Now your mob drop trap from above generates power not only when mobs fall onto it but also while they walk around before they are killed.
When you start exploding TNT near-by you're really talking some power. However, too close and you risk destroying the generator. Too far and the amount of power generated falls off. This could lead some interesting system in which you create a TNT dropper timed so that the TNT explodes at the optimal distance! I would think some variant of block placer and adjacent redstone torch would work. Maybe.
Perhaps there could be difference tiers of piezoelectric generators depending on the
material used. Since we're talking MC here and not real life the tier should be based on the rarity of the material rather than the piezoelectric potential in real-life.