I didn't mean to rain on your parade. As I said, I gave this a lot of thought some years ago and was disappointed that there wasn't a straight-forward solution. I was working with wood heat at the time, which has a much narrower range of efficient burn rates than coal (see Greg's comments).
Again, I think the flaw in this idea is using straight water as the heat-storage substance. The fact you want to work only with a range of 150 - 180 degrees makes matters worse: you will have perpetual stand-by losses on the 150 degree water, probably more than offsetting the "most efficient" burn rate of your coal.
But I still think your concept is good. Do a Google search on "phase change materials" and "latent heat storage". There may be economical materials available now that were not around when I was considering this. (Not to mention that the Internet was not available at the time, either, so researching was a lot harder.) Your thousand-gallon tank would contain the phase-change solution, and you would have coils within it to deliver the heat from your boiler, and other coils to absorb it from the tank and transfer it to the baseboard heaters.
Another type of storage you might think about -- if you are in the mood to rebuild big parts of your house -- is the Russian Fireplace or Thermal Mass type of storage. Essentially, it is a very-high-mass structure of rock, concrete, brick, or whatever, maybe as an internal wall of the house. You heat it to a comparatively low temperature but because of the great mass it stores a lot of heat and radiates it steadily back to the living area of the house. So you can produce and store the heat quickly, shut down your heater, and then use the stored heat over many hours or even days. In a sense it is like concrete-slab radiant floor heat, only vertical instead of horizontal.