I agree with Guest. More mass means more weight and the clip would just stretch or unfold. Also it will cause one side to become higher than the other. As seen in the pic there is more stress on the left than the right. Also the long leg of the clip will dig into the back every time she bends over. A hair elastic or rubber band works much better.
Girls, buy a bra with the clip built in. One, if you have small boobs, it gives them a fuller appearance(with a push up bra) and two, it keeps the straps from sliding off your shoulders
Just get a bra with a back hook.