I wouldn't worry about it too much, I'm thinking the pouring down rain will keep them away. :-D
Just joking....But my point is, if you hold an outdoor event, there are certain things simply out of your control. I think Joe's idea of talking to the Beekeeper was a good one. If the problem is yellow-jackets, there are some traps you can make for that, however, I'm not sure how much that may simply be providing an attractant and end up drawing more to the area than normal.
My guess is you're just over thinking this and worrying about a non-issue. I don't know of anyway to guarantee that a guest won't get bitten or stung by a flying insect at an outdoor event, at the same time, I don't think it's that likely to happen, but it won't be the end of the world if one does.
My older sister got married this past April at my little sisters house, where I keep my bees. She was a nervous wreck that my bees were going to ruin her wedding. I finally assured her there was a higher probability of aliens from out of space invading her wedding than the bees causing a problem.
Glad to report the wedding went off without incident...no bee problems, and no aliens! :-D