Just a thought on this but you're talking about Highland Regiment, but surely the same convention re brogues would be covered by None Highland regiments as well? Thinking of for example the Scots Guards tending to go for bulling of boots but what of their pipers? Presumably their brogues are not bulled? Also the former various Lowland Regiments such as the 1st or the Kosbies?

Should ask my Dad's Cousin as he was one, but I'm sure that the rule would be the same.

Come to think of it the wearing of brogues by officers in certain orders of dress is not confined to Scottish Regiments either...but they're brown rather than black...