I find it interesting that someone is seeking advice about marriage proposals on an a sports message board. However, people that propose at games come off as being self-centered and arrogant to many fans. Why would anyone care that you're proposing? What makes you think we come to games to see that? I think it's also unfair to the girlfriend because you are putting her on the spot about a very important and intimate decision in front of thousands of fans.
Good luck with whatever you decide to do, but I think the consensus so far is that it's a BAD idea. Sorry for being so harsh, but it's only because we care!