Could is correct. An FA that doesn't count as a "qualifying free agent loss" for the team that loses him doesn't count as a "qualifying free agent gain" for the team that signs him.
For example, Deltha O'Neal was released, so he doesn't help the Bengals and doesn't hurt the Pats. Similarly, Colvin was signed too late in the year to count against the Pats, regardless of how he became a FA.
Ok, although that sort of argument undercuts your assertion about third rounders....
To me, it's just a mental exercise, because I'm not even trying to guess what the team is going to get back for Cassel. It's such a unique situation that I'm guessing even the GMs involved (other than, maybe Pioli & BB) are stil not sure of the guy's worth.