BattlEye Extended Controls (BEC)
It runs in the background, logs into the server automatically via RCON, and then auto-locks the server whenever X amount of players join, and then unlocks them once they move out of the lobby. Also does lots of other cool things like kick/ban people for swearing, using abusive names or illegal characters, etc.
We had the lobby player limit set to 5 on ANZ1-4, and have since increased it to 6 or 8 (I forget). It's main purpose is to limit number of players that load the mission when the server has just been restarted. Too many at once, and tents/cars don't spawn.
Of course if it auto-locks whenever there is 5+ people in the lobby, and never kicks them, well the server could stay locked for ages, so that's why we auto-kick people who idle too long. Plus people would idle in lobby often to hide from bandits, or just go off and have dinner while 5+ people were mashing enter for half an hour trying to get in.
Initially we had a few problems, where BEC wouldn't unlock the server, so we just restart BEC every 30 minutes, seems to fix it.