Edit: Ah, after looking at the scripts, it looks like this is more complicated than I'd thought. There's no way (with scripts, at least) to obtain lists of players in an area, as all the waitingrooms are on the same map.
An easy alternative is to check when a player warps to bat_room if a dual-client is on any Battlegrounds map (either a match or bat_room), and kick them appropriately. That script you would only need to add once: