I think we all know that vehicles can float with tin cups, I was messing around YouTube ,and so can rocks. They are also very light when firing at them with a turret. Maybe someone could come up with a mini game with this. Sounds ridiculous, right? Not sure if this still works though..
You can't use rocks as their physics no longer update when their position is saved. Unless there's something I don't know...
Correct. In more simple terms, The only objects that retain movable physics in custom games are scenery, and of course vehicles weapons and toys. That is, however, a really cool glitch you've found and I'll be sure to add in in the Forge Tips and Tricks update.
With a crate, golf ball, die, or vehicle (do vehicles work?) it could be used in a custom game, although probably more for aesthetic purposes than anything serious.
If you'd put a vehicle in there, give it a shape and set it to be the safe haven or the hill or even something using the Insane gametype, maybe some people could think of crazy ideas.