Great points, but can't you tweak the idea somehow by also buffering individual participants? I could imagine a service here could be worth a lot more than ~$10 user.

This is only needed for micropayments so a buffer of like $5-10 could be fine if you could somehow otherwise otherwise verify the user / monetize the relationship. Best way I can think of that preserves privacy is to serve image only ads through the payment network in an appropriately boxed window every time you add a tip to the randomized micropayment scheme.