Michael- I can’t think of a better reason to be part of the local music community and in particular, to be on good(really good) terms with other harpists in the area than for the reason you posted. If you are on good terms with other harpists who play the same type of gigs you do, you can all refer work to each other when one of you is already booked and you will have several other harpists that you can call if something like you described should happen. It’s a win/win situation. But it requires that all of the harpists in your group really be equally diplomatic and professional with each other. I’ve heard several harpists tell me that they have passed of several gigs that they couldn’t take to someone else, but that person has never passed a gig to them. See what I mean?
I would suggest that you also make friends with some guitarists who can play the type of gig you do. For an outdoor wedding in particular, a guitarist could pinch hit for you, while a pianist probably could not.
For any harpist who wants to pursue this and develop a circle of musicians(harpists, guitarists, etc.) with whom you can swap gigs, or who can pinch hit for you, I would suggest that you quietly ask around as to the dependability and abilities of anyone you want to work with. Better yet, try to hear them, preferably without them knowing so it doesn’t look like your auditioning them.