It would be like an IRC server where your username is your public key - and your messages are signed with the recipient's public key, so only the recipient can

It would be like an IRC server where your username is your public key - and your messages are signed with the recipient's public key, so only the recipient can decrypt it. Like some kind of "PGP chat server". Unless you wanted Coin functionality included. Public rooms in the "PGP chat server" would have their public key as their room name. "Joining" the room would involve being sent the room's private key by an administrative bot. The server could send to all clients the full firehose of encrypted strings from all parties, and let the clients decrypt any messages they can. The clients could group chats into channels and messages by which private key successfully decrypted each string.