You can even use Objects(not static objects) 146 to 156 for effect.
Unfortunately, artillery units placed inside,will not fire. There are two exceptions.
Object 148 and 153.
Place the flak cannons in the the periphery of those objects so the flak is partially inside the object and they get active. Place them in the center and they never wake up
My testing was done inside the mission builder using the "play" command.