When I did a weeks jury service a few years back (before I got into kilts)there didn't seem to be any emphasis on dress code. One of us was a rock group player and turned up in the most casual of casual gear. A kilt shouldn't be a problem. If the accused has a problem with it, he/she might ask for a replacement juror. They can pick and choose who has the job of finding them guilty or not, though I don't know if wearing a kilt is reasonable grounds for an objection.