The basic recipe for molds for kiln-casting glass is 1 part silica to 1 part pottery plaster, by weight. Sometimes other additives are included for various purposes. Here's a link to get you started http://www.bullseyeg...tipsheet_08.pdf See page 2 for the mold recipe.
This mold formulation crumbles away from a glass casting after firing in the range of 1500 - 1600 degrees F. For your application, if the clay is already bisque fired there should not be an issue of it marring/adding texture to the surface.
Supporting a bisque structure for machining is an interesting concept and piques my curiosity. Does the machining involve CNC?