Here was situation a few months ago on Northwest.

I got upgraded on my flight from LAX-MSP and got a "F" boarding pass. My flight on NRT-LAX got in late and there was a huge delay at LAX immigration. I ran straight to the gate and I barely made to the gate before they closed the door. The GA kindly asked the person in my seat to move back to coach.

I don't know if that person was a non-rev or not, but I entitled to that "F" seat.