Seriously. IMO, it's 2001, but only if Luke Staley stays healthy (he didn't). Before that? 1996 (if BYU doesn't lose to Washington - but they did).
Do you really think a 1-loss BYU gets into the playoffs? Really? Over a 1-loss SEC team? Over a 1-loss Oklahoma/Ohio State/USC/Michigan State/FSU, etc.?
The way I see it, BYU has to go undefeated to get into the playoffs (and even then, it's kind of a long shot). That's just how it is going to be unless they get in a P5 conference.
You're approaching this as if BYU was Alabama/Notre Dame/LSU/Oklahoma/USC and the national championship was a legitimate possibility each year. It's not. There have only been two times in the last 20 years where BYU was good enough that the playoffs would have been a relevant topic. And in both of those historically great years, BYU STILL had enough slip-ups and weaknesses that they probably would have been kept out the top 4.
Boise's model, on the other hand, is much more viable (and successful). They play an easy schedule, go to (and win) BCS/NY6 games, and - if they run the table - could still squeak into the playoffs. In the mean time, they rack up high win seasons, win their bowl games (against the best team they've played that year), and build regional (and possibly national) prestige and name recognition. Their ceiling is slightly lower, but they hit it MUCH more consistently.