@karry: right now, it is mainly in the concept stage. We haven’t started properly discussing a design. Currently we are mainly waiting for grant decision.
Re zero-knowledge: I have no knowledge regarding it (in full harmony with the zero in the concept). Have some simpler ideas on accounting, but let’s see what we will choose in the end.
Re tracking usage: some APIs allow you to make as many keys as you need. So, we could assign every user one key and account using that. If impossible, we can probably make record then with allocating the keys to groups of users depending on time window. So, we can get an estimate of API usage.
Re integration: probably each service will require some specific solution on the backend. I presume that after few of them are done and tested, adding the new ones will be easier. So, it is probably major work required on getting it running with couple of services. On app side, it should be trivial - at least that is the target for us.
Re proxy: I think we cannot have any proxy between app and the service (like maps client and Mapbox, for example). there are privacy implications and it could be prohibited by services as well (not sure in the latter).