Do you really think that BYU (and Coastal) would have had the same ranking if everyone knew that the target to hit was #12? I think it gets much harder for G5s to move into the top-12 if the playoff moves to 12 teams. Regardless, BYU hasn't finished the regular season ranked in the top-12 since 1996 (but they have finished the season as one of the 6 highest-ranked conference champions a number of times) - so if we're applying the standards of today to the past, it looks like access would be better for BYU in the MWC or AAC than as an independent.
Additionally, BYU would likely need to finish even higher than top-12 (if a top-6 conference champion finishes outside the top 12, then at-large teams need to be better than top-12 in order to get one of the 6 at-large slots). So hard as it is to finish top-12, it still probably wouldn't be enough for BYU.
Independence just isn't defensible if this proposed plan is adopted.