Dan_Musicant writes:

A pool supply place I'm talking to doesn't have it but they stock
calcium hypochlorite, which he says is a "pool shocker" or some such
thing. Is that a dessicant (i.e. OK) ...

No. Calcium hypochlorite is an elemental chlorine source. You don't want
that near anything or even inside the house.

You want calcium chloride. Any pool supply place will have that. Perhaps
they're too typically ignorant to know the chemical name. They might only
know it as some variation of the words "calcium hardness increaser." Look
on the bag or tub for a chemical ingredient.