Holmoe, a couple of weeks ago, stated that going to the Big 12 wasn't an option. He didn't state why it wasn't an option, though at the time, most believed it was because the Big 12 would never add teams for football only -- some Big 12 journalists have stated this, as well.
In this case, it is likely the Big 12 and not BYU that won't do football only. The Big 12 is trying to break the idea that they are a second class P5 conference and adding teams football only would not help that perception.
From everything I've seen, BYU would jump at going to the Big 12 for football only. BYU is, overall, very happy with how they fit in the WCC. They like being with other religious schools and how that creates many areas where the schools goals and values are the same -- far different than it was in the MWC or would be in any P5 conference. Additionally, it solves the "Sunday Only" issue, as the Big 12 would have no issue with BYU football not playing on Sundays.