Mine are on soft substrate now. There is no way it would work for me to put something under the rat.
For one, they are usually primed and ready to take the rat, there is no laying it down nicely on a mat or something, they grab right out of the air. I refuse to use anything that can bunch up, they've been known to eat it along with the rat.
On top of that, they drag the rat all over the place.
I would literally have to cover the entire floor with a mat of some sort. Knowing how BP's are about changing their environment, it would cause more stress then anything, akin to removing them from their home to eat.
The only other option would be to keep them on large sheets of paper. Since I love substrate for many reasons, that isn't a choice for me.
It may work for others, not saying that it doesn't. But it won't work for mine.