Your idea is not revolutionary... people who have fish tanks (big ones) often use CO2 to adjust the PH. Normally they will bubble a small amount to keep it down. All it takes is on the order of 10 bubbles or so per minute through a special device that pushes the water down, while the bubbles go up (this desolvees the CO2 into the water, otherwise it would just bubble away). CO2 in water will not displace O2 But, if the O2 is kept up, like with a bubbler, so you can use both safely.

I always suggest using a bubbler to add disolved O2 to the water no matter. Roots just love it :dance:

Hope you enjoy keeping your PH adjusted with CO2 if you go that route. Personally, I go the chemical route. You can find a schematic of a PH adjuster using CO2 on the web by searching for it. If your good with a soldering iron, your almost there.