Chombi

Yup, I tried that and it worked to get my aluminum quill stem unstuck from the steel steering tube on my bike.
But I did not exactly use CO2 to cool it as you suggested, but a product called "Freeze-Off" from a company called CRC. It comes in a large spray can and can be bought at many auto parts stores. Not sure if they use CO2 in it but it sure really cools down the metal you spray it on. They also combined the cooling agent with some sort of penetrant liquid to help things out.
It released my stem after weeks of trying to get it out with Liquid Wrench, PB Blaster and all other sorts of penetrants plus prodigous hits from my deadblow mallet at appropriate places. It worked within barely minutes of applying it as it shrank the Aluminum stem away from the steel steering tube, cracking the corrosion and letting the penetrant lubricant in between the two parts. I even saw bubbles coming out of the seam between the two as the stuff worked it's way down into the crack on the visible white Fe/Al oxide. A couple of medium force whacks with my dead blow mallet and the stem started moving and was free seconds later. Also used it on my then seized up BB cup lockrings and they also released within seconds. I also did not have luck with PB blaster in that area.
Trick always was to aim the spray where you will get differential shrinkage of the material from the freezing. That causes the corrosion to break apart between the tow components and start to release them from each other.
One thing that concernds me is the effect of freezing on the resin used on CF frames. It might cause it to get brittle enough that you might end up cracking things if you apply force to it. So check on that before you consider any freezing solutions.

Good luck! Hope you eventually get that seatpost off.

Chombi