On second thought after studying the furnace controls I believe once the heat exchanger in the duct is hot the burner won't run, just the fan, so my solution might be to a problem that doesn't exist.
I think that is the case here. I have the same Honeywell control as you and looked at the EFM stuff and I would reccomend this. Leave all the existing furnace stuff and thermostat alone. Wire the circulator to the EFM circulator control. Jumper the thermostat screws on the Honeywell control on the EFM. When you fire the coal boiler the circulator will start on a rise from the low setpoint. If the fire dies it will shut the circulator down on a fall below the low setpoint. This prevents the coal boiler from robbing heat from the furnace when it runs. The only thing you want to watch is the temperature switch at the top of the furnace. Most of these are adjustable and you can run the boiler at a higher temp too. Between these two adjustments you just want keep the plenum temperature above the point it fires the furnace. Without connecting the two units you should have a seamless transition from one to the other automatically.
On another note, I have the Honeywell L8124 Aquastat Tech pages. I do not see any copywrite on it. Can I post it? It describes the sequence of operations performed and in what fashion, wiring options and install info. Pretty neat the way it works.
Edit: It's a good thing I went looking for this stuff. When I went back down to put the stuff in the "library", I discovered the copper tube hot water line had spung a leak. I tried to shut it off, the valve doesnt stop the water.
No water tonight. Lousy well water, eats everything.