Less complicated, it understimulates my mind!
There isn't really room left in the control scheme for a "use" button. There is a use function though which is activated when you click while there is nothing in your hands. Currently it is used to flip vehicles over.
You can try pushing it with a vehicle in a mini-game, or try using left click in first-person without any items equipped. I haven't actually tried pushing a ball, even as a beta tester, although I know they can move, because Blood Bath built an entire track for a ball to follow.