I played for a cousin's wedding two weeks ago and hated every minute of it. Here's why.
The bride (my cousin) ignored not only me, the Mrs. and the baby, but most of her other guests, too. She had an idea of when she wanted me to play, but not what. Her mother asked me for a list of tunes, which I supplied, but I never heard any response to it. There was no rehearsal dinner, which I always thought was a courtesy extended to the wedding party and guests invited from afar. The bride has yet to say, "Thanks," "I liked it," "It was pitiful," or anything at all about my playing.
All this is to say that when you find your 'piper, let him/her know ASAP what your expectations are. If you want the 'piper to pick all the tunes, say so. If you want "Mairi's Wedding" and "The 79th's Farewell to Gibraltar", say so. If you want him/her at the beginning of the ceremony and not at all thereafter, say so. And after the wedding, go over and say, "Thanks for playing for us. It was really nice."
By the way, the 'pipes are not quite as screamingly loud as I think you're imagining. If the wedding is in a big lodge with lots of over-stuffed furniture and people wearing clothes made of fabric and rugs on the floor, I think you'll be alright. You might arrange to go hear your 'piper play somewhere, and let that inform your decision.
And post pictures of the cake!