This requires client coordination so can’t just be done on relay side. It may be hard to get several clients to adopt that standard.
Also, generally speaking there is more to be paid than just the server costs anyway. How about the time spent maintaining the relay by its operator(s)?
Ah yes, operator cost too
Hmm, not really feasible then ya.. perhaps the closest one was Damus version of zap where zap sender can allocate part of the zap amount for Damus. But it's from sender instead of receiver.
Since client have more control than relay, then maybe relays that is run by client will continue to be the most basic public/free client then. And more high quality private client like wine still need to live based on subscription
