In addition,
as others have inquired, what is the official Clan stance on this issue (not simply what is "tolerated")?

Casually adopting a tartan that is not officially prescribed, by non members, is of no concern of mine. Other than the crests and heraldic devices, I'm of the opinion that the whole "clan" affiliated tartan practice is by and large a recent phenomenon anyway, at least to the stringent degree currently in place.

However, the tartan police might not take such a casual view on this, nor the clan societies in question.

How would Orthodox Jews feel if a gentile, non Levite, were to casually carry around the Torah and historic vestiges? Granted, religion vs. cultural heritage, but no less revered by both respective parties/examples.

Be advised,
I'm playing a bit of the devil's advocate here.