Why not ask about heat-resistant cement at your local builders' suppliers? You could do the casting in that, maybe.

Here's a useful thread
http://ths.gardenweb.com/forums/load/firepl/msg1021301831381.html

I'm finding my light weight 80% cryogenic perlite recipe handles heat like a champ. I've soaked it (2x8x1.25" brick)in water and placed it in a 450 degree oven and it didn't crack or explode.
I tried this with a tufa brick, peat and vermiculite, and it cracked/exploded into 5 pieces.

I plan to do further testing in the spring. I want to test how color handle direct heat like in my barbecue.

Thank you for the responses. I am always amazed at the information available on this forum. I will post my results - maybe another new application for cement will emerge from this!