Not on the ground, with the engine and climate control systems off.

I'm a little surprised to see that this is a potential issue, though, as you can buy ice cream, packed with dry ice, at the airport in Austin. I don't recall hearing or seeing anything about "please declare it to the airline," and I doubt they'd bother offering it if airlines were going to charge $200 to let somebody board. (You can also buy a to-go brisket, but I can't imagine that needs dry ice.