There are a large number of wood-fired stoves and Fireplace inserts that are ultra high efficiency and put off over 50K BTUs.  They have installed blowers that push air out to heat the home up to 2000 sq ft.  

Here is the question, how hard would it be to connect the blower to the HVAC duct (with gates to keep the direction of flow correct) and heat the house through the ducts?  

This would be a correct installation with outside air being routed to the stove and still being able to use the furnace if need be.

How difficult it would be depends on a lot of things.

It may be possible, but it all depends on where everything is located. Making it look good would be difficult. You would almost need to install the wood burning unit after designing the hookup.

Trying to jerry rig it, and how difficult that would be depends on the unit. Do you have to go through a stone hearth, a subfloor, and that damn floor joist that is always right in the way?

You going to try to hook ducting to a free standing wood burner and snake it around such that it loses all it's heat before hitting the main trunk and create a burn hazard as well?

Upstairs? Downstairs? Do you even have access to the HVAC main trunk?

Too many variables to even say yes, it's posssible.  

I've seen it done, you just have to have the stove near the HVAC unit.  A stove with a blower is a much easier ansewer.

We have the fireplace insert w/ blower and it keeps our house toasty.  We leave the house fan on to circulate the air thoughout the house.

I imagine the setup you're describing is possible if designed into new construction.  I wouldn't want to try a retro-fit.