As you said, it's obvious BK would consider jumping to the NFL for the right opportunity. The problem is that, at age 53, he's already older than the median age of NFL head coaches. Pete Carroll made the move at about age 57 and is now the second oldest coach in the NFL.
At this point in BK's career, moving to another college job would only postpone and possibly limit his NFL opportunities. For that reason, I seriously doubt he'd make a lateral move to another college job as long as an NFL job is a possibility.