I'll retract my statement. Mostly. I remember hearing it used (by Southerners) as a second-person singular pronoun but maybe they meant it in the sense of "you and all you people (from snow country / who can't barbecue / with all your teeth)". H. L. Mencken knew more about the American Langauge than any of us and said, (the plural nature of y'all)
"is a cardinal article of faith in the South. ... Nevertheless, it has been questioned very often, and with a considerable showing of evidence. Ninety-nine times out of a hundred, to be sure, you-all indicates a plural, implicit if not explicit, and thus means, when addressed to a single person, 'you and your folks' or the like, but the hundredth time it is impossible to discover any such extension of meaning."
— H. L. Mencken, The American Language: An Inquiry into the Development of English in the United States, 1948, p.337
Since "y'all" is plural, then "all y'all" is, what? More plural?