Actually, I used to supplement CO2 to my emersed plants. There are several ways to do it. You can simply run a yeast generator to the aquarium or container you house the plants in. This is a crude way of increasing, albeit slightly, the CO2. Alternatively, commercials growers use sophisticated CO2 monitoring systems that use butane (I think) and maintain an elevated CO2 level in the greenhouse.
That being said, I don't think it is necessary. Oh, and I remember Paul (HeyPK) sprinckles a few flakes of oatmeal every once in a while in order to get a little supplemental CO2.