First, I think Rex hit made the best argument.

I would remind her that for your wedding, you will be wearing a brand new kilt, a brand new jacket, and all brand new accessories. These items will only be tried on once time prior to the wedding to assure that they fit, and make their grand premiere at the wedding. Remind her that she (and possibly her maid of honor) will be responsible picking out the kilted outfit that will most compliment her wedding dress. With those requirements (and yes, you must stick to them!) I don't see how she could not approve.