OK guys, follow me on this one, I'm not sure the stack temp is the right place to do this as most conventional boiler stacks are pretty dirty and true temp readings might be hard to monitor but if you used the water temp then what you are proposing is easy to do on any conventional, I think it's a great idea and may try it at home on my old Wood Booger.
For those of you who would like to try it, this is what I would try, Place the Honeywell strap on Aqua Stat that I use (6006C-1018) on your supply line, use the R+W terminals to interrupt just the blower power wire at about 5 degrees below your set point or target temp, lets say target temp or shutoff is 185 degrees with 10 degree differential, you set the strap on at say 180, blower comes on at 175 and runs to 180 then the blower shuts off but damper stays open until target temp is reached at 185 or if there is enough load and the temp falls again to 179 then the blower comes back on.
I think it's a GREAT IDEA, Thanks Ironwood