I called the Gold Desk twice, and got the same answer twice: Travel must be completed by 12.31.2012.
Unless Missy knows or can confirm otherwise, that is our answer. There is no reason or way to 'try this out', since you have to go through the entire booking process - including paying the fare - before you get to select the option of using the MVP Gold Guest Upgrade and whether it will all you to do so.
Missy - do you know differently?