Because of your post I learned something new. The following video shows replacing a valve stem without pulling it through from the inside of the wheel. Basically you pull the valve stem into a tube and then punch it into the wheel when the tube is held up to the opening. It looks like another possible case of American ingenuity.
http://www.youtube.com/watch?v=AuwEG54yEA8&feature=related
Oops wrong video! The link shows a special valve stem funnel for installing valve stems into a wheels without dismounting the tire.