I leaned toward un-kilted when the guest/volunteer is part of the crew, but I don't think that brides ought to have a say as to what guests would wear beyond setting the tone: resort casual, jeans and gingham, black tie, and so forth. I pity the groom whose bride insists on that much control. It's up to the guests to choose attire appropriate to the occasion, and there are certainly many ways to wear a kilt (any number of which can be made low-key). Just don't show up looking as though you were waiting for your cue to pipe the wedding party in.

Upstaging the bride has a lot to do with one's behavior. If you wear a kilt, you may draw attention, but how you react to it is what's important. It is very easy to downplay your own wardrobe and deflect attention to the bridal couple. Be a wall flower. Decline invitations to demonstrate highland dance, especially after four glasses of that zippy-trippy wedding punch. Say, "I'm glad you like it. Now please remove your hand from my fuzzy tassels."

Just my 2¢,
Rex.