Is there a way to limit whoncan view threads?

Say that the party in a game I'm running gets split up into two smaller parties. Is there a way that you can make a new thread for the new party and only they can see and post in that thread? And is there a way that you can make everything happening in the original party's thread unavailable to them unless they manage to rejoin the group?

In other words, say I have six players adventuring in "Thread A". Then two the party members split off to take a different corridor. So I make "Thread B" for the smaller party to use to explore the new corridor. Meanwhile the larger group stays in "Thread A", exploring and continuing on.
It it possible to hide "Thread B" from the four players that stayed with the main group so they don't know what was down that way; and is it possible to make it so that the two players that branched off are no longer privy to what is happening to the main party in "Thread A"?

Right now, the only way to do that is to create a subforum, and limit access to the forum. We've discussed thread level permissions in the past, but it's been difficult to assess the value vs time necessary right now. When I end up moving the forums to Mongo, it should be theoretically easier to add thread specific permissions.

I can definitely see the value of it in Play-by-Post games; it just wasn't a part of the original design.