William Wallace never wore one, of course, so the whole mooning thing has to be discarded. But it was a great idea, and a great scene. Having just returned from a Highland Games event, I can attest to the appeal of a group of great, strapping men throwing things about in flowing tartan.
But that leads to the subject of historical accuracy. Are we as writers educating readers or just telling a good story? If it's the latter, do we care if we fudge a bit on the history?
Although I would never claim to be an expert on things historical, I'm bothered by two things authors often get away with: obvious anachronisms (like kilts in the wrong century) and character assassination. It may be silly, but I'd rather an author invent a bad guy than take a real person and present him or her as a villain when there's no evidence that it's true.
In the past, characters often got bad reputations from bad history: accounts like Holingshed's Chronicles that took down every story available with no consideration for whether it was rumor, gossip or lies told to villify a rival. A writer like Will You-know-who picks it up and suddenly it's forever taken as truth. For example, Lady Macbeth's kill-the-king-and-we'll-be-famous character is actually based on another person entirely, a woman who lived a century earlier.
Anachronisms are often the result of poor research, but it's hard not to think of the past as one big chunk where we didn't exist. Recognizing that fashions constantly change means that an author cannot assume that the people in Egypt always wore kilts of white linen, just as the Scots didn't always wear kilts of plaid. You have to study on it to get it right. Although whether it's Yul Brynner or Mel Gibson, the kilt is definitely a great photo op.