I think it's a dead end because now all clients would have to be patched to include this or they wouldn't work in this relay. It's probably better to have the relay host an HTML page where people can buy "credits" and then use these normally.
Later clients that want to can include streamlined buy-and-use flows for these credits by talking to some HTTP API provided by the relay which can hopefully be standardized.
A good community-spec can make this work. As communities can be proxies for relays and perform exactly the functions nostr:npub1dergggklka99wwrs92yz8wdjs952h2ux2ha2ed598ngwu9w7a6fsh9xzpc descrobes (Archive, etc...).
Makes you not run into the backwards-compatibilty trap.
And something like #communikeys gives the admins and the apps build on it the option to handle separate prices/conditions for each content-type.
You are right, but a client that has integrated Cashu wallet would solve zaps as well, so this might be a good way to give users both paid relay and easy to use zapping capability.
