I've been on and off toying around with something along those lines. It's a bit tricky (especially for me, who's not a good dev), everything on Nostr is just yeeted everywhere.
Seems like you either need to do encrypted messages like the old DMs, or just do the simple dumb method of trusting relays to only hand off events to people they're supposed to. I'm sure if somebody who knew what they were doing could get something better up and running.