MorganS:
Of course it can be used. Have you tried it? I have a dozen Due's happily receiving data on Serial with no problems.
Of course I tried it. 
I even used an oscilloscope to prove to myself that it doesn't work. It only works with the onboard USB interface.
What is your setup? How did you get it to work?
There are others who have reported the same: The Rx0 pin of Arduino Due does not work ? - Arduino Due - Arduino Forum