So... I worked out that you can do the same thing in the UOO-1 mission just by spawn-camping the alien reinforcements. I stayed until turn 103 just repeatedly gunning down the reinforcements using my Colossus suit guys who can pack an insane amount of ammo, meanwhile one of my guys ran around grabbing a bunch of stuff and putting it in the extraction area, which looks like this just before the last guy is extracted:
Aaaand this is what the kills look like: