In my native Texan English, "y'all" can certainly act as a singular polite "you" (EDIT: though it is not terribly common). I often use "Howdy, how y'all doin?" as a polite greeting to people I don't know regardless of the number of people I am addressing, though switch to just "you" after making acquaintance. Funny enough, "howdy" is a contraction of the older "how do ye" and some people still consider it both a gree…
Interesting on the “y’all”. I’m a Texan and have never addressed a singular person as y’all. If I do address a person as y’all as in “How are y’all doing?”, it’s assumed I’m speaking of the family unit that person belongs to rather than the individual. While we Texans might find it polite or friendly, in the corporate world I’ve been told to avoid using it in emails.
The singular polite "y'all" I'm referring to is generally used when the other party is not known. F.ex. I regularly, though certainly not always, hear clerks, waiters, or other service industry workers using "y'all" singularly when asking "what can I get y'all to eat" or "y'all need anything else" when speaking to exactly one person, and I use it that way myself.