Wow, for that low a BTU output you won't be pushing even a small stove very hard. A Harman Mark I like I have claims 48,000 BTU and realistically it probably can do about 38,000 in my estimation, so it could easily handle your peak load. But a domestic HW coil is not an option. The bigger Harman Mark II has a coil available but might be too much heat for you. I don't know how many BTUs the coil soaks up.
Two things to think about:
(1) As I'm sure you know from your wood stove, with a super-tight house you probably need an outside source of combustion air.
(2) Since your coal stove will be idling at low output for much of the time, the chimney won't be getting very hot (a coal flue is much cooler than with wood, typically). So maintaining draft could be iffy in warmer weather. A chimney that runs inside the house would be ideal so it will stay warmer. I never thought much about the chimney when I was burning wood, because of the high flue temps, but I find it is more of an issue with coal.
Beyond that, I don't know what to suggest for specific stoves. Hopefully someone else here has experience with an appropriate model. Too bad you can't bottle the excess heat and sell it to your neighbors.
Be sure to post what you finally find for a stove -- it's an unusual problem you have, finding one small enough.