I work for a smaller US airline and enjoy staff travel privileges - and based on how our system works, I can’t imagine this happening. There are a lot of measures in place to make sure staff travel goes just as designed....whatever LEFTOVER seats are being distributed based on the priority system the airline puts in place.
our system would automatically trigger reviews and audit if there was an offload and a non-rev accommodation on the same flight, and the agent would lose their job quickly!