I just read the Thompson book, and I know it isn't the gospel, but he discusses the great kilt as formal wear, particularly when paired with a formal jacket and accessories, as Matt has mentioned.

I am reminded of the phrase "the kilt is a garment, not a costume", and I believe this includes the great kilt. After all, formal attire with a full plaid mimics the look of a great kilt, so why not just wear a great kilt?

If treated simply as an article of clothing, I would find that the feilidh would be appropriate for any occasion, as long as the rest of the dress is appropriate.

I wonder if the historical wearers of togas wore them to church? If it was the everyday dress I bet they did.....