I dont run CO2, but I have read a little on the topic. If I understood it right, you will need a sealed room to get the most bang for your buck with CO2. If you're using a filter and then pumping the air out of your grow it would remove the CO2.
Maybe you could set a timer to scrub your room, exhaust, then turn the fan off so you could run the CO2? Kind of like a vent and purge, then reseal and refill type deal.