If you read a bit at builditsolar.com, you'll see that it's easy to distribute heat from a wood boiler through a radiant floor if you use water instead of electric (also easy to do). You pay extra for radiant heat because of its comfort, so you'd want make it your main heat distribution rather than a backup.
In the U.S. wood-fired boilers are furnaces, with an inherent pollution problem even if they are 90 percent efficient, as some are. When your thermostat calls for heat after the furnace has been idling for a long time, it will send up a horrendous plume of particulates. Efficiency dives, too. Bigger is NOT better in the case of wood furnaces. Washington state bans the outdoor variety if memory serves. Many local municipalities have ordinances that allow them to ban wood fires in adverse weather conditions. If you live in a climate and geography that traps smoke at ground level on a regular basis, you should think about this.
The pollution problem can be fixed with a large storage tank. In this approach the furnace heats the tank in a long, continuous, efficient burn. Then the radiant floor calls for heat from the tank, in some cases over a period of days. The builditsolar.com site has articles and links to suppliers. (The folks at STSS are super helpful.)
One last caution. The folks who tried passive solar back in the 1970s often discovered to their horror that they'd built a solar oven. A stone fireplace may not be enough mass if the rest of your house is carpet and wall board. UCLA has a suite of design tools based on downloadable weather history from the nearest NWS station to you. Its HEED energy-efficiency computer model is a marvel, and also free.
You have a long learning curve ahead of you. If you don't have time, you should scout up a local expert. Passive solar is super site-specific. Outside of the Southwest, generalities rarely work.

I am aware of the controversy about outdoor wood boilers. Some of the newer models claim that they are very efficient with little pollution. Of course, this is coming from the manufacturers. If not a boiler, how might you suggest heating water for a radiant floor system? We have an unending supply of wood on our ranch, and we don't want to depend on fossil fuels or the grid for our heat. If I could convince my husband to install a masonry heater in the center of the great room...but this in unlikely! He doesn't want to have to split wood into the small diameter (like 3 inches) needed for proper burning in a masonry stove, nor does he want to season his wood for 3 years.
I have seen some woodstoves that claim to be really efficient - do you know of any other heat distribution systems for heat from a wood stove beside fans? Any other ideas? I appreciate the feedback - I just joined the forum, and there is a world of information here!

We are using a specific mass to glass ratio with our passive solar design, using a slab on grade with colored concrete flooring in the great room, and we are calculating the overhangs. Thank you for the cautionary statment - we don't want to cook. We are using a local expert for this design element, and passive solar does work well in southern Oregon.

Sorry if I sounded condescending before. Sounds like you have a solid approach. Don't give up on wood-fired boilers. If you have a concrete slab with radiant heat in it, you have the masonry part of a masonry heater, and in a form that doesn't chew up living room territory like the regular kind does. All you need is the boiler, which basically gives you two choices:
1: An indoor furnace in your garage. You likely wouldn't pollute using it as backup. On my 1,000 sf slab, if memory serves, it will take about 200,000 btu to raise it by 10 degrees (84 degrees is considered the maximum). or 6 to 8 hours of the stove blazing away efficiently, after which it would go out. (No matter what the manufacturer claims, any wood furnace will pollute when it idles, smoldering, waiting for the thermostat to call for heat.) A dual-fuel oil and wood furnace would give you attended and unattended backup.
2: A foreign stove in your living room. There are literally hundreds of wood stove boilers in Europe. http://www.stovesonline.co.uk/stoves_with_backboilers.html They are not EPA or UL certified, so check with your code official and insurance company first. (They typically have equivalent EU certifications.) I bought a Stratford Eco-Boiler on eBay in Britain last spring (www.ebay.co.uk) and had it shipped over. The pound was in the toilet at the time, so I paid $2,300 all in. You'd pay $3,000 plus now.
Why there and not here? Pressurized boiler systems are standard in the U.S., and they require more control over combustion. You can get automatic damping and quenching systems in a furnace. Not so often in a stove. Vented boilers are common in Europe and hydrocarbons are expensive. Hence a proliferation of add-on stoves meant to run radiators.
Plumbed to a radiant slab, it's an nifty complement to passive solar. When you need it, it's right there. One hopes that the U.S. stove industry will wake up.
Second best is an RSF fireplace http://www.icc-rsf.com/en/rsf/RSF_Fireplaces___Woodburning_fireplaces They are super efficient, and they can connected to a central forced-air heating system. Your unattended backup would be the regular forced-air furnace.
Fans on a regular wood stove are noisy and pointless. Wood stoves are space heaters no matter what you do.
Hearth.com has no end of wisdom on wood burning. Try woodheat.org for basic info.