Are all pork in supermarket previously frozen? Where can I get fresh ones? Is there a way to easily tell visually if a meat has been previously frozen? 
Reading through the transcript on that episode of Good Eats it seems that he intended for the recipe to be done with the ice cream type rock salt:     I also like plain old rock salt. Not only is it good for de-snowing, de-icing your, your front stoop, it's good for, of course, making ice cream, and believe it or not, for baking stuff.   That being said, I would personally feel fine using the rock salt since it doesn't come in direct contact with the edible portion of the shrimp. But if I were buying bulk salt specifically to salt bake I'd err on the side of caution and purchase pretzel salt which has very coarse grains and is safe for consumption. 