What do you think of the big picture, users click on a button to donate bitcoin?
the reason i mentioned the video advertising is explained better by this:
I like the idea, but I think you should make it more interesting than a button.
Make it so they actually have to perform some very tiny task, like describe a picture in a few words, or say which version of a banner ad they like better.
Then, it is more interesting for the user, potentially even addicting, and you can collect money from the company that benefits from the work.
It would be like a charity version of Mechanical Turk...
More interesting for the user, more viral, and more money for the charity!