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 greeting _and_ an inquiry, so in some cases it's redundant, and others it isn't, depending on the listener.