I seem to remember seeing a picture back in school of a couple being married, in flight, on a Japan Airlines 747. The Captain was definitely there, remember seeing the 4 stripes on his sleeve, but can't remember whether he was simply 'presiding' or actually conducting the ceremony.
I would think for a religious ceremony to take place, the person conducting it would need to have SOME sort of religious affiliation? Although if strictly a 'legal' ceremony maybe its different...?
Basically I have no idea...