Since stoker boilers that burn bit coal are so rare, and combustioneer and Stokermatic 77 hot air stoker furnaces are somewhat common. I know this is a bit of a 'stretch'.. but:
Could you put your outdoor building close enough to the house, that you could put in an insulated air duct work, both hot supply and cold return, between the house and the outdoor building? I'm thinking that if you can find a combustioneer or stokermatic for say under $500, and they do show up for that in your area and Kentucky, Tennesee, WV, southern Ohio...
Maybe just duct hot air in, tie it into your existing ductwork, return the cool air from your ducts, and just 'super-insulate' the ducts between the house and outdoor furnace room ??
Berlin, what is the btu output of your stoker furnace ?? does this even seem reasonable??
By the way, I might, just maybe have a line on another Iron Fireman Bity stoker come summertime.. and then you need to find a boiler that is easily cleaned from the potential soot from your coal..
If I do have access to the Iron Fireman, I'll let ya know.