I have a soapstone 'heater' with a built-in oven. The oven is lined with refractory brick and is a 'black' oven, that means there is a vent in the floor of the oven that opens directly to the firebox. I fire the main chamber, fire the oven itself, rake oven coals to the firebox (which has burned down by this point) and start baking my Za's. the first one I use a raised rack, the 2nd and subsequent ones I haven't needed to do that. I have a supplemental soapstone plate I can use in the oven, haven't tried it yet - waiting till the weather cools down.
For me, the benefit from the soapstone plate will be in its non-stick properties, I think.