I think TH talked about this on the Sports Nation interview.
If the Pac12 has a contract for 4 teams but only 3 qualify for example...the bowl can select whoever they want to fill that game. What they can't do is say we want a 6-6 cougs over a 7-5 Utah as an example. They would have to take Utah if they are available.
But I believe there are a couple "open" bowls that ESPN owns that can be filled however for this exact reason. I believe the Bahamas bowl is one of them. I also think the Birmingham Bowl is another.