I tried mapping countries. I wanted all countries to map except the U.S., which I wanted blank. It took a long time to type every country's two letter code. It would have been a snap if all the country codes could be auto-filled and then I could just delete the U.S. entry.
This functionality already exists. When mapping Country, the Export Values already auto-fill in the two-letter country code. So to achieve the mapping you suggested, you would just have to delete/empty out the field value for U.S.