Honestly, some substrate shouldn't be an issue. They don't gain nutritional benefit from it, but they can digest and pass it if its fine, just like the roughage in their prey's stomachs. Other than rare instances, as long as they're well hydrated, there's no real problems. Coco coir style stuff isn't really a danger.
That said, orchid bark based potting mediums may meet your needs; the coconut husk chip kind.
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