What lens to use for an outdoor wedding/reception?

You say that you don't plan on getting in the way of the main photographer, but if I were you, I would still consider a zoom lens. I love my 50mm 1.8 and shoot just about everything with it, as I love the bokeh background. As far as asking the main photographer if you can shoot, I wouldn't bother. It's your camera and all you are doing is taking photos. I definitely wouldn't let them intimidate me into making me feel that I can't have my "big camera" with me. Just go and have fun!