After I posted about the article from the Proceedings of the Society of Antiquaries of Scotland, I idly browsed through their archive of on-line essays looking for publications about Viking era finds (the entire corpus from the founding of the Society up to 2002 is available here). There probably are some useful Viking era articles that I may yet turn up when I make a more disciplined search. However, my hit-or-miss pokings this evening were rewarded with an essay by Naomi Tarrant about a complete doublet, apparently of 17th century date, found in Caithness. It is plaid, but more of a windowpane plaid than the sort of plaids many people associate with Scotland.
The citation of the article is: Tarrant, Naomi. "The 17th century doublet from Keiss near Wick, Caithness." Proc. Soc. Antiq. Scot. 131 (2001), pp. 319-326. It can be downloaded here (scroll down to the article title about two-thirds of the way down the index list).
EDIT: There have been other 17th century garments found in Scotland. A write-up from Proceedings of the Society dating back to the early 1920s about another doublet found very close to Wick may be downloaded from this page.