If the altar will be well lit, perhaps you can get by with just exposing for that part of the scene? It would look like an outdoor night wedding, for sure, but isn't that what the B&G planned? Not to be pedantic, but if the B&G really wanted all their guests to be lit up during the ceremony, why did they plan an outdoor wedding at night? You could of course use all your OCF and speedlites during the formals and reception. Any chance of asking them those questions without scaring them about what the photos will look like? I did a wedding in Cyprus last May and it was an outdoor ceremony at night. I used existing lighting during the ceremony, with a little fill flash for some shots, and the darkened sky and ocean behind the B&G looked great. Shot it mostly at 1/40th at 1600 ISO (any higher on a 5D and it's just way too noisy, imho).