Winner stops before winning
The lead runner at the Bucktown 5k, in Chicago IL, stopped just short of the finish line, walked to the side of the road, and presented a marriage proposal to his mate.
He wanted to show her that winning the race was not as important to him as being married to her (he finished 30th).
The manner in which marriage decisions are made vary. This one provided the race with a newsworthy event and one that could garner the race additional publicity, albeit not until the race was over.
Couples have chosen to wed at many events, some opting to exchange vows at high profile races like the New York City Marathon.
If you are aware of something unusual happening at your race, publicize it (unless it's supposed to be a surprise). If you obsesrve something interesting that you didn't expect, take pictures, interview people, and capture the activity for future publicity.
It may not always have something to do with running, but if it happens at a running event and people want to know about it, it has great promotional value and you should exploit that.
Don't worry about appearing crass. The race will benefit, the people involved will benefit, and others will think the whole thing is pretty cool.