Try it on your own truck. Take the front belt cover off so you can see the belt, set your brakes to on and turn both front wheels forward by hand and watch the tensioner. It will flex up. Becuase it is flexed up, it creates an outwards angle and encourages the belt to run off of it as it travels forward under braking tension. Hope I explained it properly lol.

So, unless the belt is heaps loose to allow for lots of braking belt skip and less pressure on the tensioner (so it doesnt flex) , it will most likely come off under brakes.