There is a principle of physics called the conservation of energy, sometimes expressed as the "first law of thermodynamics" and remembered by many of us as "there ain't no free lunch". If you get that fireplace to heat any air it will be at the expense of the radiant heat. A Rumford is the best solid-fuel radiant heater ever developed. There are lots of better air heaters. I don't think I'd make my Rumford heat air nor try to plow with my Jaguar.

Having said that (rather sarcastically) we have been playing with an air circulating system shown at left. We had in mind preheating make-up air for the fireplace but you could blow air through the channels in either direction and take room air rather than outside air through the intakes.

Best,
Jim Buckley