A fired mold, interesting idea.
As with mudslinger any molds I have made in the past were all plaster. They always gave me trouble with releasing after the clay had dried to leather hard, that and because they don't give as the piece dries they also caused a lot of cracking if I wasn't paying attention and didn't take them off at just the right time
I do use foam rubber forms sort of as a slump mold to let some of my thrown and altered pieces 'rest' upside down on (the ones I have problems with warping and sagging of walls or bottoms) and find they are fine for that.
I'd be interested in hearing your expereince with your bisqued mold when its ready. Keep us posted.