I think I understand how subsectors work. I'm going to try ReX's method tonight, as soon as I'm done taking care of something IRL.
In the meantime, I have a second problem. I started work on a munitions factory level, and one section has a bunch of rockets spawned on a conveyor belt, as actors, with no speed whatsoever, so that they are carried along the conveyor. I tested it, and the player cannot get close to the rockets without dying horribly. Is there a way around this?