How so? The Big East champion gets a guaranteed BSC invite, but Notre Dame football isn't in the Big East. Potentially, ND and the Big East champ could play each other in a BCS bowl.
Not talking about the BcS.
UND has two deals. If UND has a record and rating that equals the lowest BcS automatic qualifier, it gets 1 of the two "wild card" BcS slots. Nothing to do with the Leastleftovers' slot. UND esentually is "stealing" one of the wild card slots.
If UND does not make the BcS, the Leastleftovers has, like every conference, a strict progression of bowl slots based on conference finish. #2 goes to the Champs Sports in Orlando, #3 goes to the Car Care in San Antonio, #4 goes to the Pinstripe in NYC, #5 goes to the Birmingham in Birmingham or the Liberty in Memphis (the years alternate) and #6 goes to the Beef O'Brady's in St. Pete. BUT, unlike every other conference, UND participates in the mix with the Leastleftovers. Wherever UND would have finished in that league (actually somewhat more complicated than that, but lets just go with that for this discussion) UND "steals" that slot and eveybody in the Leastleftovers moves down one.
Now on to BYU. Could BYU make a similar deal relative to the BcS? Sure. If BYU belongs in the BcS one year, they will get it one of the wild card slots.
But what if it does not? Every bowl has a strict deal just like the one I described above. #2 goes here, #3 goes there, etc. The only "open" slots are when a conf does not have enough winning teams to fill its slots (ex: the Big 12 has 8 deals, but only had 7 winning teams last year, so its #8 slot fell to an "open" and the bowl took a MAC schools, since the MAC had more winning teams than it has slots).
So, BYU goes, say 10-2. But does not get in the BcS. Without a special deal with some conf, BYU would fall all the way to the bottom of the bowl world, playing in some pre-Christmas nothing bowl, since those are the ones that get freed up by the big confs not filling their slots.