Here's a cheaper option: Replace the motor with a Trip-Saver motor and add a relay to switch between the lower-HP capacitor.
Another option might be to add a motorized bypass damper so that the air within the ducts will recirculate more, thus lowering the CFM and the humidity level.
And another option might be to add a motorized damper that will simply cut the airflow down. Actuators can be set for different amounts of opening thus the damper will not completely cut off the air flow if properly set. Use a spring return actuator (Belimo if you want really good quality) and set it for power-close/spring-open; This way if the motor fails it will not affect the airflow. The return is the best spot for dampering air flow.