Seems to me the problem is that the dress code is vague enough to omit anything. That part about "clothing not usual for the workplace" is tough. So rather than work with the rules, work with the people.

Remember that bureaucrats are terrified of using their own judgement, and HR is as bureaucratic as they come. The HR director will probably assume that the kilt is inappropriate because she believes that others will think that. She needs proof to the contrary, proof that can't come from emails.

So wear the kilt in to work. Expect that by lunch the HR director will tell you, apologetically, that you should go home and change.

Here's the important part: When this happens, wince and say, "Oooh, I'm in the middle of something really important that can't wait. [Insert technobabble here.] I'll take care of it as soon as I can." And then go back to your work.

"As soon as I can" CANNOT be before 4:00 pm.

By then, there should be a consensus in the office. I'm pretty sure the consensus will be that the kilt is not harmful. The consensus will find its way back to the HR director. So, the next week when you say to the HR director, "You know, the kilt seemed to go down well last week, and people are asking me when's the next time I'm going to wear it in to the office. I'd like to wear it in on Fridays." Once you get your Fridays, getting your Mondays through Thursdays shouldn't be a stretch.

DISCLAIMER: I work in a lab where full pants are required (even for the women), and have not tested this theory.