Nice idea and, as usual, great execution Niel!
But we should pay attention to move the discussion outside the plain kind:1 model, because some clients could loose content. I don't know how zap replies are structered, but I suppose it's possibile to make them standard-reply compliant, maybe simply using two ["e", <event-id>, "reply"] tags. 

 I don't know either haha 😅 so good point sir.
But I could imagine ways for them to be readable by all clients using some kind of tag combo indeed.