like, you could add a prefix, like PGP does, which has a series of versions of the secret used in the symmetric encryption of the message, for each recipient, this will be a fair bit of extra data but it's doable

you could also have a special message that comes through a DM that the client scans and the client would have to decrypt it automatically and it would have a header that indicates it is a cipher for decrypting private events from you

there is threat models to both of these that are relevant as well, and neither of them are as private as a live peer to peer delivery and that's impractical due to intermittent network connections (mobile devices) and general unreliability of networks

and the last piece is what is the purpose of this, exactly, because it is a lot of rigmerole to just have a private status message or something