A bit further to the OP's question "is tweed traditional as kilt fabric", there is the mid-19C drawing of John Brown, of course, but no verification that that drawing was a true portrayal and not just a sitter's, commissioner's or artist's license.

We do, however, have five-generations-old official photos of The Mackintosh welcoming HM King George V, and the Prime Minister Lloyd George, to Moy Hall in September 1921. The Mackintosh was wearing a tweed kilt with matching jacket and waistcoat -- a Highland suit. That seems to give some sort of right to a claim of tweed tradition and acceptance to at least some level of "matching".

Peter I know you prefer to think in terms of 18C, but you may have a youthful memory of Captain Stuart Davidson's oft-worn tweed kilt in the 1960s (?) and 1970s to which he had added a watch pocket inside the left waist band, a key pocket inside the right band, and an under-apron pocket accessed from the right (proving that nothing is new under the sun).

If we accept the Brown portrait, do The Mackintosh and Captain Davidson adequately provide 150 years of "traditional" tweed kilt bridges to today?