Hmm.. that still leaves a lot of options open. E.g. is there prior info to work with? Would you be able to lookup "profile info" that gives a hint or provides some info, then you need less of an announcement message. I think you are aware that the basic encrypted DMs derive a single fixed key per pair of users.

I know it's not exactly what you asked for, but I played around with a small experiment which I hadn't had time to follow up on. Maybe it can give you some inspiration. (It didn't receive much attention at the time.)

I had explained it here:

https://lists.cypherpunks.ca/pipermail/otr-dev/2024-October/002574.html