This is an interesting question. The pH-KH-CO2 chart is likely for a constant 25C (78F) temperature. As you said in your first post, non-reactive gases (N2, O2) dissolve better at lower temp. Since CO2 does react with the water to form carbonic acid, I'd assume the effect is less pronounced. However, the lower temp will also change the CO2 equilibria. Without more research (read: when I'm at work with my reference books), I can't really say more with any certainty. At the worst I'd suspect the lower temp will only help, so get to 30ppm CO2 by the chart (pH-kH) and you should be at or above 30ppm at the lower temp.
72g bowfront planted, CO2, 4x - T5HO, Eheim 2213 and 2217, 2 angels, pristella tetras, blue tetras, betta, albino bristlenose pleco, albino cories. Sword, vals, hygros, ludwigias, java moss and fern, anubias
2g Mac-quarium. Clown gravel, fluorescent plastic plants, and 2 guppies.