They would just have to be one of the 3 best at large teams.
Well, in 2012, ND would have earned their spot, if that is how they would want to continue. Boise State would have made it in 2011, despite losing 1 game. TCU (pre-Big 12) would have easily made it in 2010. Cincy (if you consider the BE wasn't a P5), TCU, and Boise would have all made it in 2009.But that would be based on votes, and the voting would always be biased against them, so it would be practically impossible. They could beat everyone in their league by 50 and beat a couple P5 teams on the way to 13-0 and there would still be voters who would vote them out based on their name and league alone.
Well, in 2012, ND would have earned their spot, if that is how they would want to continue. Boise State would have made it in 2011, despite losing 1 game. TCU (pre-Big 12) would have easily made it in 2010. Cincy (if you consider the BE wasn't a P5), TCU, and Boise would have all made it in 2009.
Lot of chances IMO.