I've thought before about giving forum badges (or maybe even forum BTC credit) for donating x BTC to Bitcoin-accepting charities, but it's difficult to track. There's no way to prove that you donated to the EFF, for example, as far as I can tell. It'd be nice if there was some standard protocol for this sort of thing.
That's exactly what I had in mind too actually, although it would be hard to track as you say. Well, perhaps we shouldn't shut the door on this one just yet
.