Ewan--I do not know if you are open to suggested amendments to the story behind one of the songs in your collection, if you are the following may be of interest

Sir Walter Scott in his comments on old customs and songs was rarely at fault, he having a wide circle of correspondents to assist with any question that arose which he could not answer from his own resourses, however his formative years were spent in the only part of Scotland where the gaelic language was never in use and this may have led to the mistake he made when commenting on the song "Silken Snood"
The clue to the meaning of parts of this song lies in one of its other titles "The Puin O The Breckan". Breckan {in its various spellings] is pure gaelic and has no connection to the plant bracken, it being in fact a woven woollen cloth used in such garments as plaids and kilts.
The Black Watch, formed around 1838, wore the Breckan Dhu, the black plaiden.
We learn that the waulking of the woven woollen cloth was, at least in the South West of Scotland, referred to as PUIN THE BRECKAN and this usage would fit better with the rest of the song.

Joe.