I see no problem, Disney is perfectly capable of queuing people. There will be no attractions, besides the meet-n-greets, so once people have met the characters they will tend to disperse. There will be no more loitering around like there was in ToonTown (anymore so than there already is on Main Street.)
Also, with the MnG's on MS people will have more of a tendency to do this first, or sometimes before the parades, instead of waiting to the end of the day when most people are trying to leave the park.
I foresee no problem with the queue interfering with people trying to exit the park.