How about a small UI at the far right of the screen, above the map, that indicates whether or not you are in an overflow server? It also allows you to control your queue status. You can select to exit a queue or enter a new queue. When your queue comes up, you get a smaller version of the timer bar and a choice to leave the overflow is highlighted. Perhaps it can also contain an option to enter an overflow, if you are in a city.
The most important part is that there are no more pop ups!