Thanks for the extra input. The concept sort of exists, but is not easy to understand or is accessible for most people imho. The closest app was Pact in 2012-14, which had huge success, but now appears to be abandoned.

This is really great input. I agree that the charities would have to be delicately balanced in order to be motivating and tasteful at the same time. There are a lot of ways it could go wrong.

What would you think about, instead of the charities having the money go into a bank that you could earn back over time. Say after a month you've paid $50 in penalties, each time you're active you earn a portion of that back. Also, let's say there's attrition on the funds over time, you only have so much time to earn the money back before it's gone for good? This is a 4th option that's been coming up after talking to others.

p.s. another thought could be that your friends can steal your bucks if they're hitting more of their goals than you?