Read the end of the second paragraph in the History of the Tartan at www.kinnaird.net/tartan.htm and you will see that breacan is given as the gaelic word for tartan.
After explaining Patersons puin o the breckan before singing the song in the early 1970s the late Captain Angus Russell B.W. informed me that for generations gaelic speakers had referred colloquially to the Black Watch as the Breacan Dhu due to the dark tartan worn by them. I am not aquaint with any current member of the B.W. to confirm this statement but am sure that a note will exist of this usage at some source.
Once again I would reiterate that Patersons usage is more in kilter with the chorus of the song than that of pulling bracken:--
"And twine it weel ma bonny doo
And twine it weel tha plaiden
The lassies tint her silken snood
In puin o the breckan."
Joe.