Coconut husk (brick) is a great substrate, you can control humidity very easily. I wouldnt use it to mix with hay. Hay tends to mold when wet/damp for a period of time. You can seperate them and that would work. I use coconut brick sand mixture (50/50) for my 2 sulcatas and it works extremely well. Its easy to clean up after them, and you can grow grasses and plants in it (with proper lighting). I feed fresh grasses that I grow, and I also grow them inside the tort table on a certain rotation.