Tim. I'm wondering if lining the hot face of your casting with firebrick would allow you to use foamed Portland cement, or "Foamcrete" or "Aircrete" as it's popularly known. Foamcrete is Portland cement that has been mixed as a very wet slurry and then a detergent or foaming agent is added and then the mixture is whipped up with a drill and a mixing paddle. You can also get or build a foam generator, make the foam, then mix the cement slurry directly with the foam. I built a foam generator, and it works well, but my experiments with foamcrete have not gone that far yet as at the time I was focused on creating a foamed ceramic refractory material.
I have seen experiments with foamcrete in RMH use, but always where the foamcrete is directly in contact with the flames, or just behind a thin wall of metal ductwork. Even where it was used in high heat contact, such as in the riser, it held together pretty well, mostly only suffering surface deterioration. I would think for the outer bulk layer of your casting it may hold great promise. It is lightweight, extremely insulative, and very inexpensive.
I encourage you to look into it.