Applications of the UI

I'm unfamiliar with the internal workings of the UI system, so I figured I'd ask and let someone who actually knows what's going on answer my question. Are UI elements able to cause in-game effects? For example, could you add a toggle above the hotbar that grants improved movement speed? If so, could that toggle be only made available to certain characters (such as only Barbarians and Monks)? If this is possible, how would one go about implementing it?