That's a clever idea about dimming the lights to run the dehumidifier. However potentially it may not supply the dehumidification you are looking for because if you are running AC to cool the room then you will lose the dehumidification power of the AC while the lights are dimmed. so the quest will work much harder to achieve dehumidification in that period without the AC helping out. You could combat this with a propane/natural gas/ethanol heater in the grow room (equal in BTU to your lights). So if you are running like 5000W in lights then you have 17,000 BTU/h to make up for when you dim the lights. So heater like the one in the link below would keep your AC going while the quest does it's thing.
Keep yourself warm when enjoying your favorite outdoor activities with the help of this excellent Mr Heater Vent Free Blue Flame Propane Heater.
www.homedepot.com
an even better product would be able to turn in and off in automated fashion to be on with your Dehu similar to the propane CO2 generators which can take a 120v input for signal to start and stop.