Don, clay molecules are flat hexagonal discs:
http://upload.wikimedia.org/wikipedia/commons/e/e7/Clay_magnified.jpgThese discs, when squished against each other, overlap and provide flexural strength, much like steel plates on a suit of armor. In slip cast earthenware, such as the pot you're working with, the clay molecules are suspended in water, and, when the water evaporates, the molecules are pointing in every direction imaginable. It's molecules akimbo and produces the weakest possible ceramics.
Flexural strength is directly related to thermal shock resistance. This is why cast materials, such as fibrament, require a metal deflector in a grill setting. It's also why pressed/extruded materials, such as cordierite, can handle direct flame.
Earthenware is especially susceptible to thermal shock because of it's propensity to contain air pockets- air pockets with a different rate of thermal expansion than the clay surrounding them. As your setup stands now, you've got parts of the pot that are exposed to direct flame, while other parts are blocked by the plate. It's not a question of if the pot will crack, but when. You could block the entire pot from below by using a larger plate, but that would only exacerbate the top to bottom heat issue.
I think this is a worthwhile idea, but I would suggest finding another material for the dome and steering clear of earthenware in high heat settings.
