Actually, dry ice in small quantities (a few lb.) is FAA permitted as an exception subject to some simple labeling requirements. See here. I don't think they've modified that yet, though I doubt screeners would appreciate it (as they did not appreciate my water ice at MHT).

Prior to the current insanity, I had several times carried dry ice and ice cream as carry-on, often with amusing effects but never with hassle by TSA. See here and here .

If we were able to establish that (water) ice were not a prohibited item, then we would be able to carry 5 lb. as dry ice to use as a coolant to keep the "potential liquid" from melting. Of course, quenching my thirst by licking a dry-ice cooled block of water ice (which will have itself cooled to about -78 C and would cause all sorts of injury to one's tounge) seems only slightly more appealing than drinking untreated water from an aircraft lav sink.