I think ultimately the solution is for the Humble people to talk to Google - and to Apple, for that matter - and say "we do this thing, it makes a lot of money, how can we do it through your servers?". If they can get the owners of the official marketplaces interested then something will happen.
They could add a slider to control the percentage of your purchase prize which goes to the App Store owner. Except you can't adjust the slider, since it's fixed at 30%. Ouch. I'd much rather continue giving those 30% to the EFF.
