I agree sliders would be overdoing it. But what about penny, nickel, dime, quarter options. Of course, they'd have to be abstracted for an international audience, but make it analogous to coinage, spare change, something tangible that people intuitively relate to. I agree that you don't want too many options. You want just enough flexibility.

I think a really innovative approach would be a pledge/rescind model. Let people freely pledge up front up to a certain liberal limit then present them with the registration, payment, tedious paperwork all at once after the fact. Let them welsh out on as many pledges as they want. What is a pledge that is later rescinded worth? Less than a penny obviously but more than nothing at all. I think most people would willingly agree to follow through on their donations even with given the opportunity to back out, if the details weren't too burdensome at the outset.

The problem with every microcurrency scheme I've come across is that it enforces the kind of rigid transactional exchanges that fit traditional commerce models when, with a little imagination, they could do very interesting things with the networking and computational possibilities available through the internet.