as i see it, to make it viable for as many people as possible it would have to be able to feed the larger sizes bituminous, have an easy way to deal w/ the fines and have a large, sloping ring around the firepot to deal with the chunks of coal that fuse together and burn casually on the side of the main fire pot. do all these things and you will likely have a winner, don't successfully do all of them and it will continue to be a poor selling fringe market for the boiler, with the only purchasers being the people that are willing to put up w/ more work to burn bituminous.
on edit i'm not familier w/ the efm boiler, but it needs some access to horizontal areas of heat exchanger as there will be some soot over time. during development don't expect to be able to produce completely smokeless results esp. with common high vol bituminous coal, ideally most underfed stokers burn w/ a light grey haze, (big ones or small ones) if one tries to eliminate all trace of smoke, you will end up burning out the pot in short order.
btw, if you need a supply of decent bituminous coal that is high vol (like most of the country's coal) contact mark at valier coalyard, valier pa (outside punxsuatawney).