as far as i can tell, you could do this with a fully custom client, and the posts would just be jibberish to all other clients
not useful from a "keep posting to your same social graph" perspective but very interesting from a "start a new social network, while piggybacking on other people's nodes/infra" angle
alternatively could be approached thru mini apps pretty directly too, but that brings the content off-chain so it feels like cheating
-- {no channel}