At the risk of sounding pedantic, an elite who purchases a ticket within his or her upgrade window would clear immediately if X or R inventory was present when the ticket was bought, assuming the upgrade request was put through at that time. I've had this happen before, it literally goes right to X in the Manage My Booking view and gets an F seat. Not exactly "instant" in that it doesn't book into a first class inventory (like a normal instant upgrade fare would), but more or less instant.