Resurrecting this, in the same boat, did you find a workaround? I know World at War has nearly the same console/command setup and people can join in and out fine. I'd like to know what makes it work, either a command or series of commands.
It'd probably work all the same if you put it in the bo1 console, again, whatever makes it work for World at War can't be too different if you put it in bo1.
And for the "xupdatepartystate" command. Since it updates the lobby as soon as you press start, I'd like to find a way to block that command from running. As your guess seems to make sense.