Something like serverCommand only executeable by the server (no remote execution) to lock/unlock server by script.

e.g.:
Currently once the headless client logs in, my server loads the mission from the config Mission of the server config and the headless client is assigned to a slot.
A few seconds later the mission is started.
From server start till playing state it takes about 3 minutes, but if a player joins before playing state, the mission won't start if the player hangs around in the lobby.

Having a script command to unlock a server once the mission is running would be a convenient solution for this without a need for an Admin always.