You should be able to get added to the waitlist once at the airport - but on long flights your chance of not having a MVP in front of you waiting for a comp upgrade is little to no chance, so pick a good coach seat

just as an example, I do sea-mia and sea-mco all the time and as Gold I have a hard time getting comp upgrades lots of times so I know there are a ton of MVP behind me that never do get the upgrade, meaning anyone with no status surely has never prob gotten one

ps: see this related post
http://flyertalk.com/forum/showthread.php?t=562968