Option 1 - Wrap the fan with an AirLoopHVAC:UnitarySystem object. It is a multipurpose system object which can be set to use LatentOnlyLoadControl. It should cycle the fan on whenever there is a latent load.

Option 2 - Since you already have an EMS program in place, change the sensor to "Zone Predicted Moisture Load to Humidifying Setpoint Moisture Transfer Rate".