Tennessee (30-0) and Alabama (29-0) went undefeated in non-conference play in 2007. Those were the last teams to do it.
Unfortunately, I do not see the Cajuns' RPI being high enough for a national seed. If the Cajuns go undefeated and a lot of things go right (meaning Cajun opponents finish unusually strong), the Cajuns might, might be able to swing an RPI of about 10. But probably about 11-13 in that scenario. The RPI simply favors the stronger conferences too much once conference play kicks in. The Cajuns may be at their zenith (w/ respect to the RPI number) after this weekend. After that, it will be a desperate attempt to hold off the elite conference schools that will be charging hard up the RPI rankings.
Brian