I'm pretty sure CO Insider has gone on record here saying that the waitlist disappears 24 hours prior to the flight. Prior to this time you do have priority over free elite upgrades, but after this time, including at the airport, you do not.
Some have reported success in getting Gate Agents to assign them higher priority at the airport, but I don't think it's supposed to happen anymore.

If you search a little you may find the thread. Try "upgrade waitlist miles/mileage" or something.