Need to control fan speed at extremely low level
I know it's not really an electronics solution but it sounds like maybe you could just use a large hood with a constant ventilation rate over your smoker. 

As long as the hood exhausted at a rate equal to or greater than the exhaust rate of your smoker smoke wouldn't back up into your apartment.  You wouldn't need to worry about burning chips too quickly because excess flow for the exhaust would come directly from the room. 

Instead of a large hood you could maybe accomplish the same thing with a draft hood like the kind used on gas hot water heaters.