https://en.wikipedia.org/wiki/Flattr figured out some parts of this. Notably, you picked your own total monthly donation, and then clicked a button on participating sites to allocate a fraction of your total to them. AFAICT it worked as advertised, but raised new issues with donation behavior. E.g. I obviously like curl every month, so should I click its button monthly? Twice monthly? If I am a developer of some other useful OSS software, should I click curl's button and the curl devs click my button? Does the money just slosh around between merchant-customers? Is that good?

I believe the idea has been attempted many times before, primarily by large companies that have tried to create their own currency. It seems deceptively simple, but it's quite tricky to get right, both from a legal and technical perspective. One of the main legal complications is the one mentioned in another comment: avoiding the status of an e-money institution.

With Small Transfers:

  - There is no wallet or funding for the account. Customers simply pay for what they owe, usually at the end of each month.
  - There is a lower psychological barrier, since there is no subscription or prepay commitment. Customers who dislike recurring payments are more willing to try something new that avoids this.
  - Merchants need to introduce customers to just one extra service, Small Transfers.

Some customers of Unattach (a service I built) are happily paying for the service via Small Transfers, and early feedback shows that they really appreciate this pricing model. It's worth noting that Unattach also supports the classic subscription model.

As more merchants adopt Small Transfers, customers will still only need one account, making micro-billing even more convenient.