Since you won't "MATCH" anyone else, you will probably be fine with whatever you choose. I may be wrong, but since you said coat tails, the men may well be in WHITE tie, in which case, I would suggest that you bring out your full artillery.

On the other hand, you like to be understated, so your usual armament may need a little boost. I like MoR's suggestion of a 3 button waistcoat, though I suspect your tartan one is more likely to be of the 5 button variety. Would you consider a jabot?

If you are the only one in highland dress, most people will have difficulty sorting out what is formal, what is kilt, and what is wedding officiant. Someone will undoubtedly want to know the ecclesiastical significance of the color of your flashes.

Thinking just a little more, and trying to balance the full on formality with your low key role and approach, if you won't go with the jabot, maybe the best idea is a solid black four-in-hand tie and a regular collar. Keep us posted.