Hello Mike yes several of us are using a boiler with a water-to-air heat exchanger in the hot air ducts for heat.
From a practical standpoint, I'm not sure you would ever pay for the conversion to a boiler with heat exchanger just to keep from burning propane for domestic hot water during the summer.
What does your propane hot water heater use during summer months?? Then you have to subtract the cost of burning coal to keep the water hot, and a big boiler is not a real effecient way to heat a small amount of domestic hot water.
You will be keeping a whole boiler full of water hot, all day and night because you can't let the coal fire go out every few hours.
If you use a lot of domestic hotwater, such as for lots of children and clothes washing, it may pay off, but I'd consider looking into solar heating a tempering tank and or an on-demand propane water heater.
I've looked into keeping my boiler running all year, but the propane is only about $60/month for domestic water, cooking and clothes dryer, and the all important barbeque. [it's hooked to the house system].
I have all the items for a solar system, I'll get around to hooking it up one day, I just don't want the solar collector on or near the house. I will get over the looks of the collector when the fuel bill drops enough.