At one time I was trying to figure out a way to have make a manifold of copper tubing that would circulate water through the outer (wool) insluation of my kiln while it was firing.
Rather that use that radiant water heat immediately, thereby losing MOST of the heat loss to no effect, I wanted to try to have the circulating water come from a buried tank, the mass of which could actually retain the heat for longer term use to heat my shop.
Thing was though, my shop is incredibly efficient -- we use pet groomer's hair dryers to dry glaze on pots whose walls are thrown too thin to absorb the excess moisture. Those hair dryers can actually warm this shop -- it's that well insulated.
And the cost and effort of trying to capture the kiln's wasted heat would probably cost more in time, and the gamble that it'd be worth it, than the yearly expense of heating the shop. Plus, I try not to fire too much in the winter when the local utilities have raised the natural gas prices. Thus, I'm not really firing the kiln when it would be needed to heat the shop.
still......
(hey....speaking of "still".......all that copper tubing....and that heat.......and the cornfield across the street....