Hey, super secret tech I uncovered for making the Ballista objective suck less: If you start a match, then tell the herald to stop the match after the opening ceremony (once you've gotten your "run and hide phase" sneak/invis), that counts as a completed match and you get the clear in like a minute rather than ten.
You can also start a match with only one player inside the diorama zone. Not sure if you need to have at least one other player previously registered, didn't test that far, but basically you can have one person set the zone up and then people can just march in and out setting up and cancelling matches nice and easy. There is a bit of delay in the system declaring a the master of ceremonies role vacated, so it's like one every few minutes and not just as fast as the cutscenes will allow, but still, pretty smooth turnaround.