A lot of modern Shetland tunes have jazz influences (thanks to Willie Johnson and his radio tuned in to Schenectady) and Tom Anderson was a professional music teacher and pretty well educated about a lot of genres. I see no reason why he couldn't have meant that.

I've never played harmony on that tune but I've often heard something I didn't expect or understand going on in the background when doing the melody - a lot of Edinburgh folk guitarists have been influenced by Jimmy Elliot, who was doing the same sort of stuff as Willie Johnson at around the same time, but died quite young and didn't leave many recordings.

It would not be a good idea to pull out something like that unless you first made sure that anybody else playing an accompaniment part knew it was coming.

Actually the second fiddle part just has a held B flat note there. It occurs to me that it might simply be an A sharp written enharmonically, which would make the chord a first-inversion F#.

Anybody play F#min there?