In the October 1803 issue of the Scots Magazine (available in Google Books), Robert Jamieson published a list of 20 ballads that he was interested in finding versions of for the collection he was planning, with some comments on each one. The list includes the following:

"12. MAY COLVIN. I think the Ballad on this subject, which I have heard when a child, was better than that which has been published.
13. CLERK COLVIN & THE MERMAID. Of this I think as of the last mentioned. There is another tragical Mermaid Song in Scotland, of a more extravagant kind, which I should like to procure."

So apparently Jamieson (born in Morayshire in 1772) vaguely remembered a version of Child 42 from his childhood which was different from what he had seen in print (by which he presumably meant Herd's version, 42B, and perhaps also the altered version of 42A in Lewis's "Tales of Wonder"). It doesn't prove much, but it at least raises the possibility that the ballad had more life in NE Scottish tradition in the late 18th century than the small number of surviving texts suggest.

And I wonder what the other "tragical mermaid song" was?