None of the clients I know can handle pay-per-event model.
Discussion
Keychat is a chat app, built on Bitcoin ecash, Nostr protocol and Signal protocol.
Keychat uses ecash sats as stamps for messages. 
Ah, I wasn't aware. Really cool!
What would a client need to do to make this work with zaps (as opposed to #cashu?
The relay can send "messages" back to the user in the client, right?
Yes it can.
Not yet ...