You seem fairly resolute that knowing if someone has their butt on their seat is useful, but I would invite you to try finding some other metrics that actually

You seem fairly resolute that knowing if someone has their butt on their seat is useful, but I would invite you to try finding some other metrics that actually measure output, and try rewarding people for performing highly on those metrics. I can tell you, I've seen countless unproductive hours wasted with butts firmly on seats. It's not a good measure of productivity.