They haven't expired. When I bought my Signature Edition, the codes said that they expire on the 31st Dec 2012. (PAL)
Also, there is a Chromeline Jaguar XJ13 you can get for free: https://www.gtplanet.net/forum/showthread.php?t=135391.
Edit: Just saw you are from the US. Yes, they will be impossible to get. Sorry.