That's a very good question, one which many people were asking today, and one which I really don't know the answer to. One the one hand, BYU has a policy of not playing on Sunday. They clearly would never have scheduled a game for Sunday.
On the other hand, BYU also believes very strongly in honoring its commitments. Having integrity is a commandment just like keeping the Sabbath day holy is. And BYU had a commitment to play the game as it was scheduled. That strongly implies a commitment to FINISH the game. And because of the partnership with ESPN and their broadcast, this is more than just a commitment to the other team and the people there to watch the game.
It would actually have been kind of a lose-lose situation. Either way there are MANY people who would have complained.
Personally I believe the commitment aspect would have won out and they would have completed the game. But who really knows?
I think about 5 years ago I read about one of the BYU teams (volleyball?) that had a similar situation, albeit without huge numbers of fans and no TV broadcast. The game went late on a Saturday night. If I recall correctly the coach let the players decide what to do, and the players voted to stop playing and consequently forfeit the game. So again, who knows what would have happened this time.