I imagine there may be some degree of difficulty in programming something quite like that. Case in point: Valve's multiplayer console games. In Team Fortress 2 of the Orange Box, anyone on your player list can join "private" games while it's not so in my experience of either Left 4 Dead game. So I just put two and two together and assume there's some trick with it that isn't well-known amongst 360 developers.
Heh, I was playing Perfect Dark Zero online and people who weren't even on my recent players list joined my so-called private game. But back to join-in-progress, if the admin has kick powers in private games, then it'll be a non-issue, surely.