I've seen springtails raised in a charcoal + plaster of paris medium at universities, and I don't think it had anything to do with trying to get nutritious springtails. Rather, I think it was supposedly the medium that enabled them to produce springtails most efficiently/effectively.
I'm not sure what plaster of paris consists of, but I'd be surprised if it wasn't calcium based. Whether that calcium is useable, I don't know either. I keep meaning to try mixing some of this into a few of my cultures, but never get around to it.
Food for thought...
Josh