I'd almost suggest using a footnote to denote: "usage may vary in different locales". :(
I originally asked a question about an expression (you know) and not y'all. After aubie convinced me not to use it in the singular, I looked it up on Wikipedia and now I'm convinced he was right.
There is long-standing disagreement about whether y'all has primarily or exclusively plural reference. The debate itself extends to the late nineteenth century, and has often been repeated since. While many Southerners hold that y'all is only properly used as a plural pronoun, strong counter evidence suggests that the word is also used with a singular reference, particularly amongst non-Southerners.
Notice the "non-Southerners" which is me. I didn't want to come across as stupid in my story.
H. L. Mencken recognized that y'all or you-all will usually have a plural reference, but acknowledged singular reference use has been observed. He stated that plural use 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 Supplement 2: An Inquiry into the Development of English in the United States, 1948, p.337
Note the "he stated that plural use is a cardinal article of faith in the South" and "ninety-nine times out of a hundred, to be sure, you-all indicates a plural."
So I took it out of the story.