I too have considered casting borosilicate. I am searching for a kiln and recently passed one up that was capable of the temperature and control but was smaller than stated.

I am not sure about the Boron oxidizing out, I don`t believe it does. There are other less stable compounds that will oxidize out and it was my understanding the the melt temp. would go down with a radical increase in co-efficeint of expansion. If your assumption is correct then the glass would oxidize to quartz solidify and anneal all at 2200°F and under. Many refractories use gas heating rather than electric to stablize the oxidation process.

This leaves the question of starting material. I believe if one uses the raw materials it will require a batch reactor that is stirred to obtain a homogenous mix and then poured. If one used broken PYREX then expect many bubbles and inclusions. PYREX frit is probably the method of choice but I am not sure you could save anything in the way of costs.

I am eager to hear what you figure out and can probably come up with the annealing formula once you get that far.

Good Luck,
-Rick