I would second the sanitation and contaminant concerns, but you might be interested in an experiment my son and I ran a couple of days ago. Someone sent us steaks in the mail that were kept frozen with dry ice. I put 2 gallons of water at around 60 degrees F in a 5 gallon bucket of water. I threw in the 2 by 3 by 6 inch piece of dry ice into the water while outside and my son standing 15 feet away. No explosions. No overflow of water. It bubbled CO2 steadilyfor about 15 minutes until it was gone. I took a temperature reading when it was completely sublimated (turned from a solid to gas). The temperature read 50 degrees F. I was thinking the water would turn to ice and totally did not expect that we would only see a 15 degree F drop in temperature with just 2 gallons of water. So even if we thought that dry ice was sanitary and didn’t contain toxic contaminants (which we don’t), there’s no way you could add enough dry ice to cool wort at 212 degrees F down to yeast pitching temperature. Wort, being a sugar solution, would contain a lot more heat energy at 212 degrees F than straight water, so it would take even more dry ice to cool it down. Finally, the viscosity of the wort and the hot temperature would probably cause a massive foaming and overflowing mess everywhere. So it’s a no-go any way you look at it.
