@JesusIsGodAlmighty @JRay
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.
If you have a javascript app for chats, implementing private chats shouldn't be too hard - in fact your hardest objective would be optimizations and real-time support (Flarum atm is fairly static and doesn't refresh the page by itself).
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.