If you already have forced hot water, why don't you look at replacing your oil fired boiler with a coal or a coal/oil fired boiler? Keystoker makes great units. The KA-6 is their most popular unit and with the dual fuel option, going away on vacation is no big deal. In the summer, if all the boiler is doing is domestic hot water, it will burn one hopper every 2-3 weeks. The other alternative is to run a coal fired boiler in series with the oil fired unit(provided the oil unit is not about to give up the ghost), having the coal unit do all the work, and the oil unit as a back up system only. The plumbing is not too difficult(it is rather easy, just time consuming), and then you have the best of both worlds. The Keystoker boilers put out 200 gph of hot water, and are built like a tank. The KA-6 weighs 1000 pounds. I just installed one for a customer last week and he is loving it.
I have an Alaska Model 140 auger feed at my house(I have forced hot air), and it is tied into the existing ductwork, and my propane furnace is now the back-up. I also installed a hot water coil into the stove, with a circulator pump, and it works great. When the 140 is under load, you can run cold water in, and it comes out real warm after one pass. I have a 50 gallon hot water tank, and it is childs play for that coil. Stokerstove is right on the money with everything he said. I don't think that a hot water coil will do anything significant with a baseboard system. The KA-6 unit holds 55 gallons of hot water for the baseboard system, so it has plenty of oomph to get things done. Inside of the hot water storage is the domestic hot water coil which gives off endless hot water year round.