Mackey should get into the Hall if for no other reason than that he took on the league itself and won, breaking important ground for all of the players (and their contracts) who followed after him. Put him in the Enemies of the Establishment Wing, if you want, and throw in Al Davis—who also missed the cut last month—for good measure.
Whatever, John Mackey should be in the Hall of Fame. It's that simple. If you visit the place, you want to see his bust. I remember thinking that exact thing as a kid, studying my football cards, late at night.