I got engaged on the field before the Florida game in'08. The University won't let you do it so you have to find a personal hookup. I think they do rent the sky boxes for special occasions if you want to throw down some serious cash.
My Sister had some of her engagement photos taken on the field. She is an UT Grad, so that could have played a part. They also painted the rock, as said earlier many people do. Maybe you should paint the rock to ask the question. Taker her to the game and tour the campus. Then go by the rock and pop the question.