molding and casting research

I conclude that yes, hydrostone and drystone (which mas.863 has lots leftover) can be used for metal casting.
DRYSTONE Fast-casting gypsum cement that provides strong, durable casts without the need for dryers. Excellent compressive strength and good
impact resistance. Enhanced flexural properties. Low absorption for reduced paint demand. Environmentally preferred to resin-based
alternatives. Available in solid or hollow cast formulas.

HYDRO-STONE Especially suitable where high strength and resistance to wear are necessary. Used in high-quality art and novelty castings, molds, figurines,
modeling material, anchoring material, density fill applications, lamp bases, and die production. Works well in most mold materials, especially
flexible molds. Expansion about two times greater than Hydrocal A-11. Self-leveling when poured and not suitable for hollow cast
DRYSTONE – Fast setting high strength product that needs no drying. An environmentally sound alternative to resin-based products. No hazardous chemicals or waste, no styrene emissions and no required environmental controls.
HYDRO-STONE – One of the hardest and strongest of all gypsum cements. When mixed properly, has a heavy, syrupy consistency ideal for pouring solid models or patterns. Recommended for stretch-press dies where extreme surface hardness is required. Slightly higher setting and expansion than ULTRACAL 30. Used for high quality art novelty and statuary castings. 
plaster is used for metal casting but requires drying for several days
gesso – Plaster or a fine plaster-like material made of gypsum, which is also called whiting, used for sculptures. An especially versatile medium in reliefs, gesso can be either a material cast in a mold or a material of a mold, a material to be modeled, or carved, or attached to something else. When used for molds into which molten metal is poured, it must be hardened with sand as a grog. Gesso may also refer to such a gypsum material mixed with an animal-hide glue and used as a ground for painting. For this latter use, it isWEAR A DUST MASK! usually applied to the surface of a wood panel or sculpture to become the surface on which an artist paints. It was used by Gothic and Renaissance panel painters, and is still used today. Oxgall (or another wetting agent) can be employed MEDICAL ALERT!to eliminate pin-holes in gesso surfaces by mixing it into the gesso before the gesso is applied. Like all other dusts, airborne gesso is hazardous to breathe — every user should wear an appropriate dust mask. Also see slip and stucco. 

therefore gesso is probably food safe.  so if I coat my pink foam model (which can cause sensitivity with prolonged exposure — cue architecture student horror stories) with gesso and then cast it with food-safe silicon, should be good.

overview of molding techniques

discussion of 3d-printing plaster molds,37834,3783