In this hypothetical, we're talking about a 12-0 BYU. That would mean BYU beat Boise on October 24 in Boise. Assuming Boise is otherwise undefeated, they would only have games against UNM, SDSU, Wyoming, and Utah State (another team BYU would have beaten) left with which to try and jump back over BYU.
I'm guessing you're new to college football and therefore just didn't realize how unrealistic that would be.
Having beaten (in this hypothetical 12-0 season for BYU) BSU and USU, two of the top teams from the MWC, and also having beaten UConn, Houston, and UCF (two of which are expected to compete for the AAC crown), a 12-0 BYU would be in absolutely zero danger of being leapfrogged by a G5 team this season.
I will concede that the November schedule, or lack thereof, most likely rules out a top 5 ranking, though.