...in recordings by white singers, such as Woody Guthrie, it's "we" that hanged Stackalee, while for black singers, it's "they" that hanged the bad man...

Well, Mississippi John Hurt was certainly black, and in his version we find the lyric:

Gentlemen of the jury
Please don't fight and fuss;
Don't you know WE'D better kill him
Before he kills one of us?

The exception that proves the rule?

Of course, now that I think of it, any jury in that time and place would have been all-white. So I guess MJH is "quoting" some hypothetical white folks saying "we."