I think that when one is referring to Highland attire one has to differentiate between style of dress, and the occasions when certain modes of dress are customarily worn. It is this fusion, in my opinion, than creates what some may characterize as "tradition".

So, when attending a wedding in Highland attire the expected mode of dress would be kilt and sporran, tweed jacket, hose, white shirt and tie.

Likewise, when attending a tartan ball the expected mode of dress would be a doublet or coatee, dress shirt and tie or jabot, kilt and dress sporran, and dark or tartan hose.

Now both of the above occasions suggest what is "traditionally" worn, but without addressing the style of the apparel, which could be anything from very conservative to something off the catwalks of Edinburgh (if such exist!). Which leads me to the point of this posting:

I believe that "events" are easier to categorize as "traditional" than attire-- "We traditionally celebrate Burns' Night on the first Saturday in February, and we customarily wear black tie to the dinner." sums it up nicely.

As far as the antiquity of Highland attire is concerned, by and large the clothing worn today is hardly changed from what was worn in 1900, and certainly it is virtually the same as that worn in 1930-- as is most men's attire. In my opinion one is headed down the wrong path if one looks to pipe bands or Highland regiments when discussing customary civilian attire; uniforms are just that-- uniforms-- and no variation in dress is allowed, unlike civilian attire.