All things depending, I would wonder if you'd have to pose this to any boss at all. I worked for a company in the past where the union partly negotiated the dress-code for office employees -- this was reviewed with me during my orientation... didn't say anything about a kilt, so I asked, and the manager/s said the dress code didn't say anything against it, technically I could go for it.

Consider this...
Are there other people who wear heritage clothing to work, religious ethnicity clothing ... if you have Scots heritage, how is it any different, and therefore why should you have to ask or prove anything. I think the more important question/s should be asked in a setting like this forum -- what would be the correct combination of kilt-clothing to meet office standard.