I have no experience venting coal dust, however I used a Fasco 480 cfm blower to vent a bench top conformal coating work station vent hood. The fume hood was connected to the inlet of the squirrel cage by way of a home made flange adapter. It worked great and since the motor is isolated from the vent path there was no concern for fire or explosion.
The unit is available at the following link:
http://www.electricmotorwarehouse.com/f ... blower.htm
It is available in both 220 and 115 VAC versions depending on your electrical service availability. If you constructed a duct which attached to the top of the hopper and spanned the entire width, you could effectively suck any dust as it rose above the pile of coal in the hopper. Also since you would be extracting air right at the source of the dust I would think there would be no concern for a negative pressure in the area of the combustion air induction on the boiler.
Just an idea.
I may try a variation myself to solve my bride's concerns for the stove in the house.