That's tough. I know that I, personally, would NOT want to be surprised by a public proposal. I think there's a big pressure to say yes because
-you're suddenly, and unexpectedly, the center of attention
there you are, enjoying a sports game, when you see your face on the jumbo tron.
-there's audience encouragement
Think of how much catcalling and whooping there is during a "kiss camera" where people who look like couples are put up on the jumbo tron and encouraged to kiss. When they do, there is chering. If they don't, there is jeering.
-you don't want to create a scene/make people feel uncomfortable
when the answer is yes, the audience gets the warm fuzzys of seeing a tender moment. If the answer is no, it's just awkward.
-a fear that the proposee will be heckled for refusing
Along the same school of thought that you should always accept a date because the poor person mustered up the courage to ask you. The poor guy/girl went to all the trouble to arrange this awesome proposal and that heartless Female Dog/Bacon Fed Knave ripped their poor heart out.
If you're going to say no, I think you should say no right away. Logistically, imagine the proposer had told friends/family to watch the game and they all think there's a wedding to plan?
More importantly, it's cruel to dash false hope. Here the proposer is thinking that there's a wedding to plan and a future with the love of his/her life, while the reluctant proposee is trying to figure out how to unaccept.
Yes, there may be some awkwardness, and some backlash from the public at the unexpected response, but that's on the proposer, not the proposee. The proposer is the one who is causing the situation. Imagine you're at a party when a guest admires your jacket. Guest then demands the jacket, and makes a scene. You can sense that Host wishes you would just give Guest the jacket to keep the peace. You aren't wrong to not give Guest your jacket, and the scene/discomfort isn't your fault.