I refused wearing the kilt because if I wore it that day no one could take it seriously any other day. Therein lies the issue, I think.
It is the issue. IMO, if you want to have the kilt treated like any other clothing, then treat it like any other clothing. The kilt seems to be the only type of clothing (as far as I can see) that offends just for being worn as part of a costume. That, in and of itself, prevents it from being treated as any other type of clothing.

Since you are discussing the issue in the abstract, not the merits of any particular costume, I think the parallel between someone renting a kilt and PC to dress up as a "Scotsman," and that same someone dressing up as a German in liederhossen and a Tyrolean hat, or as a Frenchman in stripped t-shirt, beret and cigarette, or for that matter a European dressing as an "American Tourist," with loud shirt and camera. While all involve stereotypes (as do many Halloween costumes for sale such as "hippie" "goth" "punk" etc.) none carry the racial baggage, and history of discrimination, as a white person dressing in black face.
Thank you! This is what I was trying to say (but not nearly this well).