Q,
Your point comes through loud and clear. Hugill OFTEN opined without real evidence. You don't have to convince me -- after all, I'm the person who thinks the "John Kanaka" story is bull :)

Luckily, in his first book (as opposed to the later ones), he lays out all the information he has (scanty as that may be), so that when we does opine, it is fairly transparent to the careful reader that he is just trying to make some sense out of things.   When one is deeply familiar with a whole body of songs, one starts to have a bit of an intuitive sense about how they are related -- a sense that won't come to someone else who just looks at one songs and tries to make conclusions based on the immediate evidence. So I am glad that he included his opinions, in an effort to highlight possible connections, origins, etc. I would have been disappointed if he only wrote "I don't have any specific evidence so I won't say anything about this.

With "rig a jig," like with "good morning ladies," you are of course right that it could be used in a variety of English setting by different people. It is not so much where it could be used though, as where it did happen to be used, in this case in a minstrel-ish chantey ("Clear the Track, Let the Bulgine Run"). And that suggests they may share the same idiom -- enough to give an opinion, in lieu of "hard" evidence. Mind you, this is not a scientist's "opinion," as the humanities rarely offer that kind of hard evidence. You have to say something!

We can say, "Hey Stan, this chantey is not necessarily of Black origins, since you cannot prove that for sure," but I think he knew that -- he makes plenty of disclaimers to that effect. If we have an opinion that the chantey is of another origin X, then we can say that and offer reasons, but we'll be subject to the same criticism of "not necessarily; no hard evidence." So I think positive reasons to suggest (!) an alternate scenario are also helpful.

Gibb