[ note that I was editing the post above when you replied]
I'm no wiring expert, but afaik, the least compicated way to control a part time 'booster" fan would be to parallel wire it into the stoker feed motor circuit. It would run whenever the stoker fed.
Let's say that it took the stoker 3 minutes to load the stoker bed with coal after the triple 'stat called for heat in the boiler. Assuming for a moment that it would be good to delay the booster fan, a "delay on" timer could be used. This would also prevent the booster fan from running when the keep fire timer ran the stoker. I don't believe extra air would be a benefit during those short feed cycles.
Here are some delay timers; http://www.ebay.com/sch/i.html?_nkw=del ... m270.l1313
In case of dead link, go to ebay, search "delay on timer", click us only box.
You can find all sorts. The smallest will handle only an amp or maybe 1.5, but a small axial fan might not pull even that much. You can find timers that'll handle bigger loads, but they are pricey.
This all said, it does seem to me that for hard running, the bigger fire likes a tad more air than it needs on idle [watching the draft signal on my water gauge tells me so too]. The bigger Keystokers are set up as such. I am surprised that someone hasn't already tried this on a Kaa-2.
A real goofball could also use another timer to limit the time the booster fan runs. Once the bed of coal is "roared up", the extra air might not be needed. One could also rig up a temp switch to cut the booster fan if the flue gas temp exceeded a certain point, etc.