Currently, all we have is fof/byobu. Unless something changed in the last week, there are no instant messaging extensions that feature private conversations that work for beta 12.
But to be fair, Byobu, ignoring some privacy issues with unencrypted data in the database and the flagging system, is already a pretty good solution. Unless it's absolutely mandatory for you that the system is end-to-end encrypted, you should use that extension, otherwise delegate private messaging to some other service, i.e. Telegram, WhatsApp etc.