Pretty good idea!!I use a large plastic(round) container, put holes in the top of the lid. I put that inside my bp's home, to keep it warm. I put the mouse/rat inside the container for a few min, then I add my bp. It only takes him 5-10 sec. & he has already got the mouse/rat. Then you don't have to worry about it eating any substrate.
If it was a small piece of substrate, then it should be ok. I am not sure, but cypress mulch isn't good to have as substrate.
Aspen, newspaper is much better - just some ideas.