I suppose a tank-style heater wrapped around the sides of the stove would work well, but then you get into licensing and insurance issues since it will be a heated pressurized vessel.
I built a waste-oil heater inspired by the Mother Earth one, but I used a 25gallon propane tank as the casing, with an Army surplus 40kbtu tent heater instead of their half-assed attempt at a burner, with a heat exchanger from an old steam pressure washer mounted near the top of the main chamber. A circular baffle plate spaced down one inch from the top edge slowed the escaping exhaust and allowed more heat to remain, then a 4" stack out from the top center. I plumbed the coil to a car radiator with an electric fan, and a 12v circulator pump completed it. Roughly 20% anti-freeze mix, since western WA didn't get that cold, and I had a heated garage.
You could possibly adapt this to pre-heat water, as long as it's circulated, but again you're into the heated pressurized vessel thing again. Not a big deal in a detached garage, but in a house it generally is a big deal.