Rain!
For one thing, no matter what you think or what the forecast for 3+ months away is, we rarely get rained out. Yes, it's true.
However to answer your question, what I would do is stand inside in the room on the side where the outdoor ceremonies are with the doors open and take pictures as if you were outside. I've done this so many times (at other places) and it doesn't even look as if it's raining (when it's done right.) The other spot would be under the overhang looking out to the fountain where the cocktail hour might have been. Or upstairs at the balcony.
Of course check with your photographer about what they would suggest, but that is what I would do.
But don't worry. You'd be amazed at what we can do in between the drops.
