I had never heard about NASCAR teams doing this, but I certainly wouldn't doubt it. I can answer a few questions about cryogenics though, as I have worked in the air separation industry for over twenty years.
Cryogenic liquids are produced by a distillation process which separates the components of air by their different boiling points. Liquid nitrogen and liquid oxygen are the two most common cryogenic liquids. Liquid nitrogen is the colder of the two and boils at -321 deg. F at atmospheric pressure. This is what they would use in this application, as liquid oxygen can be quite hazardous, especially if it comes in contact with oil or grease.
I suppose one of the concerns here would be the alloy being cooled. In our plants we have to use either aluminum or stainless steel piping for any low temperature parts of the process, as carbon steel piping will crack or even shatter when exposed to sustained temperatures of -40 deg. F or lower. I would be hesitant to experiment with a cast iron block for this reason. A metallurgist could probably answer this.
As far as doing this as a "home project", it would probably be impractical. The cryogenic liquids are generally delivered to bulk customers (hundreds or thousands of gallons) by large tanker trucks that are vacuum jacketed similar to a large thermos bottle. It is actually a tank within a tank with a vacuum in the annular space. This vacuum provides an excellent insulation for the inner tank. The liquid nitrogen itself is really not that expensive.
I do know of a few plants in the Pacific Northwest, one in particular being a BOC Gases plant in Vancouver, Washington. These plants can usually be found listed in the Yellow Pages under Industrial Gases or Oxygen. Some of the major companies in this field are Air Products, Air Liquide, Praxair, and the aforementioned BOC Gases.
I have a good friend who is the Plant Manager at a plant in Vacaville, CA, and he is an avid gearhead (mostly Pontiacs). Maybe a "clandestine" project could be arranged
I would volunteer, but I'm currently working overseas in Malaysia until next summer. sometime.