I like the overall idea in theory, but as others have said, it's all in the implementation. I think the UI will be very important on the page that controls the contribution to the artists. I think you should have 2 "shortcut" buttons - "Pay Supported Artists by My Play %" and "Pay all artists equally" and then have some sort of vertical equalizer sliders with each supported band having a slider representing a custom contribution setting. I think that would work ok for people who want to tweak their contributions, at least for people with a small amount of supported bands. You could also specify a time frame for the first button... Year/Quarter/Month etc. The main question... what is the default setting? Is it equal to start, based on play percentage, or does the UI force you to try out custom contributions at first?

I don't think having users putting in percentages is a viable option. Alternatively to the aforementioned ideas, I think a 1-5 star ranking system would be useful, and then the percentages would be derived from that, on the backend. (with a FAQ and such available explaining the % correlation)