Although it might not be practical for widespread use, I think a reasonably clever person could retrofit their system with a means to inject CO2 or Nitrogen into the chimney. CO2 is much easier to come by these days, with all of the places refilling paintball gun cylinders. It could be manual or automatic with a heat sensor/trigger. The proper heat sensor would be the trick to find. I know such a device could never be approved by any regulatory agency, but the worst that would happen (short of blowing up a cylinder, etc.) would be that it would fail to function, and if your were relying on it to extinguish, you'd be up the creek. Actually, if you didn't absolutely rely on it, the worst that would happen would be to have a false trip, putting out your fire. We inject CO2 into silo fires, it should work fine for this application, if someone could make a reliable delivery system. Of course, this really overcomplicates things, but I love a good contraption!