PY standby could be tested to see if they even turned that functionality on just by booking a close-in flex ticket and looking at the result before the 24hr refund period expires.
I am more curious what it looks like on a route like yyz-dxb, after we have been waitlisted for J cabin and answered YES to the concierge who asks if we want them to notate we are willing to take a PY eupgrade if J ends up full. Are we ever on both waitlists? Or do they manually review the "didn't make it into J" list before closing the flight?