I have also started feeding in the viv, and also use cypress mulch. when i started to do this i was paranoid of him injesting the substrate, so to try to avoid this or keep it to a minimum i defrost my FT rat in a plastic sealed baggy, then hit it with a hair dryer on low heat to make sure it is dry then dance it in his viv using long tongs. He is such a good eater it barely touches the ground, so far its worked well. I really don't ever see much substrate on it, just a little dust pieces, and his poops seem normal and weekly.