Oh, "you people" has its use down here too. Usually with racial/class connotations. Definitely something you use to denote group endogeny/exogeny (i.e. you are one of
those people, and I'm not). And it has such a history of being used condescendingly towards blacks that you can start a good fight just by using the phrase "you people".
I thought about it some more, and I think I figured out the whole "y'all as singular" thing. It functions similar to the
ba particle in Mandarin, in that you use it to 'soften' a sentence (by referring to the person in the plural, you're not
specifically referring to them, even though from context you're totally referring to them).
Examples:
(trying to tell someone to leave without further provoking them): "Y'all just might want to turn around and walk out, afore this gets ugly."
(consoling someone without being too..."clingy"): "Y'all gonna be okay?"
EDIT: Incidentally, this is reminding me of a long-shelved conlang project I had, where the "elder race" had to come up with indirect pronouns, because their power was such that the very act of speaking changed the world. So they developed a set of 4th-person pronouns (sort of a "he/she/it who shall not be named" thing), and lots of grammatical fictions like this so they could talk
about someone without actually affecting them. Yay for linguistic nerdism!