Likely BYU would be somewhere in the low teens of the CFP rankings and MAYBE the high single digits. So the would no question be the top non-P5 school. However, there the playoff rule states the highest ranked G5 team gets a NY6 berth...and BYU is not considered a G5 team as far as the playoff is concerned.

It's quite possible BYU is either passed over and Marshall or CSU gets the bid (BYU would have beat BSU, so they would be out); or BYU gets an at-large invite in addition to a G5 and those two are matched up against each other in a then devalued Fiesta Bowl.