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.
This Blue Flame-20,000 BTU Liquid Propane Vent Free heater is the perfect supplemental heating solution even on the coldest days. This heater is conveniently equipped with a thermostat for superior control
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.