I might be wrong but I don't think you can control CO2 levels using some kind of soluble carbon pills. Also, I would be worried of the effect of such tabs on the cultured cells.
What you can do, is try to control the pH using a buffered medium, so that cells are not affected by the lack of incubation. While exploring our live imaging options, I came upon the Opti-Klear buffer from Marker Gene Technologies that looked interesting and might help solving your problem (I have not used it, though).
Thanks Andrea! I was also thinking that the tabs will maybe produce effervescence or currents that may affect the cells. Our medium is buffered with CO2 (without it it changes the color and becomes basic)
I think HEPES is indeed the easiest and effective solution. We use 20 mM HEPES in our live cell imaging experiments. Check out HHBSS (HEPES-buffered Hank's balanced salt solution) which is often used in live cell imaging.
As the others have mentioned, HEPES-buffered HBSS with glucose is the best way to go. You can also overlay the medium with a small layer of mineral oil to prevent evaporation, etc. Ensure your temperature is still controlled, however. This should be sufficient for 24-36 hours of incubation.