I heard you can use the 'tree' looking plugs that a lot of cars have as fasteners. I'm going to try it this weekend, bought a new bumper off here and it has the front license plate bracket. Going to take it off and see what I can plug the holes with. I'm not spending $150 to fill 4 holes if I can help it.
Check this thread for 'tree' pieces -
http://www.camaro5.com/forums/showthread.php?t=38988