I would dress as I would, wearing slacks... You can always lose the Jacket and tie if you find that you are overdressed compared to the other guys.
So... Kilt, hose brouges flashes, squian dubh (if you wish) evening sporan (if you have one - day sporan otherwise...no horse hair) kilt belt. Short sleve shirt with tie. Day jacket. Skip the vest. I don't think that head gear would be required here. As a piper I usually wear the Glengarry...but I have done weddings bare headed. Depends on what seems right at the time.