As long as the soil is organic, and doesn't contain any pesticides/fertilizers, I'm pretty sure it should be fine.
The best way to make certain that it doesn't affect your cham is to cover the soil so that the cham can't get to it. Some people use large rocks/pebbles to cover over the exposed soil (make sure they are large enough that your cham can't try to ingest them).
I use fine plastic garden mesh (the kind they use to attach creeper plants to vertical surfaces) cut to shape over the top of my pots and cable-tied down. It's not the ideal solution, because if the cham leaves droppings on it, they sometimes fall through the mesh, making it harder to clean. But I find it easy enough to remove the plastic every so often and give them a thorough wash whilst cleaning up the topsoil in the pots.
just because it says organic doesn't mean anything.
It has to be OMRI listed to be classified as organic.
I know miracle grow sells an "organic" potting mix with time release ferts in it
honestly in my opinion soil is soil. even with a small amount of time release ferts, its not going to hurt your cham as long as you keep the cage clean, keep the top layer covered with top soil , hydroton, or river rocks.
also keep in mind the type of plant you are planting and its requirements
you wouldn't want to put a ficus plant in a mix with low porosity as they don't have much water or light requirement , where as some plants to need constantly stay damp so if you used the same mix, you would need frequent waterings and vice versa