Sometimes it is best to think simpler. I had a great idea that works quite well.
Go to a hardware store and get 20-25" of 1/2 copper pipe. Solder the bottom closed, fill with water and add a little salt to keep the freezing range low, solder the top closed. slide this down your tower and rest the bottom on the keg, so that the metal is touching metal. The top should touch the metal for your tap and run the entire length of the line. Thanks to the specific heat capacity of water and copper, you will have a cold beer line
-n