Thanks for the input. I'd been thinking about crushed coral and oyster shell, but wasn't sure if it would have unacceptable levels of sodium. Though I've read that some people water certain Lithops species with brackish water every so often...?
I'd been thinking about Pseudolithos flowers and difficulty of obtaining seed (not that mine are anywhere near flowering size) and wondering if one couldn't use a fine long haired paintbrush to do pollen transfers? I've not seen the blooms in person so for now, I'm just dreaming about it.