When I tried to burn West Virginia Bitum coal in my Iron Fireman, which was set up like this EFM, that is suspended over an ashpan, without supporting refractory around the firepot,,, what happened is exactly what Chris described. The tall coke tree grew, and when it fell over, the bottom of the coke tree was the fire, and the fire went with the tree into the ash pan...
From the description of the Wyoming coal I've read, the EFM design should work, it may not be as efficient as burning anthracite,,, but at ~$50/ton... who cares??
With no clinker removal needed every 12 hours,, this boiler should be a winner... nobody likes to be tethered to their heating appliance... I'm really enjoying my AA boiler in this warm weather,, my hopper holds about 2 weeks worth of coal, and an ashpan is good for about 3-4 days... virtually maintenance free.