Well, this specific example could be solved with a "exlicit" tag (clients/users woul need to set it though). But I suppose you are thinking of a more broad solution? Some days ago I read about relay sets: You define specific relays for a specific content and switch them easily in your client to change the view.
Discussion
That's a good idea. I think whenever someone posts, maybe there should be a UI knob of some sort indicating where they want to direct that post: to my "wall"/"microblog", "to community X", etc. In this case "to communitiy X" would be a set of relays configured in your settings somewhere. Maybe "chat with my family" is one of them.