I heard an official say that, once a ball is tipped, there can be no interference.
I remember this from 2006. In the LSU-Auburn game, there was a plethora of bad calls, but the one that applies to the Johnson-Barron pass defense can be found around the 3:20 mark. The Barn is called for pass interference and then the flag is picked up because a defender knocked the ball away.
http://www.youtube.com/watch?v=ESgoMBi6d3g