It's been awhile since I worked on my furnace. The more I thought about adding a hot water coil to the furnace plenum , the simpler this may be. Here's some additional thoughts.
As I mentioned in a reply above, the fan limit switch controls the blower and the burner motor by sensing hot air temperature inside the plenum.
When the wall thermostat calls for heat, the burner kicks on and starts heating the plenum. When the air inside the plenum reaches the temp that the fan limit switch is adjusted to, it kicks on the blower.
As long as the air in the plenum remains above the limit switch low temp set point it will make blower continue to run.
When the wall thermostat reaches the set point, it shuts off the burner, but the blower keeps going until the fan limit switch senses the air in the plenum has dropped below it's lower limit set point.
If you add a hot water coil in the plenum before (below) the limit switch, that coil will keep heating the air thus making the limit switch keep the blower running.
If the wall thermostat is set just below actual room temperature with the boiler and blower fan taking care of the heat load, the burner won't come on unless the coal boiler output drops, thus dropping the room temp near the wall thermostat, which will turn on the oil burner.
If the hot water coil can add enough heat, without restricting the air flow too much (squirrel cage blowers don't like air flow restriction), it should be able to keep the blower fan on, and the burner off, while supplying heat.
The will run none stop as long as the boiler hot water coil is hot enough, but the electric draw is not as costly as the stop/start plus the cost of fuel oil.
Here's more reading on the fan limit switch. http://inspectapedia.com/heat/Fan_Limit_Switch.htm