I do not use Y'all, but I have unquestionably heard Y'all used in the singular. It blew me away totally. One memorable instance involved the utterance of a woman from San Antonio, Texas. Another, a woman from Alabama. Both were Ivy League educated, both instances were perfectly spontaneous, and both were followed with a strong affirmation that Y'all can be singular.
Both my wife and I have used "ya'll" all of our lives. It is always plural in our circles. The singular would be either "you" or "ya" as in "How ya doin?" The possessive would be "ya'll's". We were both raised in Central Texas.
I'm from Kentucky, Louisville specifically. In my experience y'all is recognized as "Southern" style. Even used in Boston, it would be thought of a Southern dialect usage. Unhappily, Louisville is ever confused about where it is: too north to really be southern, but clearly not Yankee. Not really mid-west, but not eastern or western either. We are ambivalent about "y'all" too. My observation is that native Louisvillians use "y'all" exclusively as plural, and avoid using any possessive form, reverting to "standard" English like "yours". (Your's ??) But this gets muddled by the non-natives, which outnumber the natives, and speak every imaginable style and accent. Interestingly, I've found that people speaking English as a second language pick up and freely use "y'all" very quickly. Most languages seem to have a plural for "you", and “y'all†fills this void very easily.
It's intereting how everyone knows what Y'all means and recognizes it as a Southernism, even if they never use it themselves. When my wife tells people that she's from Texas, they tell her, "You don't have much of an accent." Then, if she drops ONE "Y'all" into the conversation, they say, "Ohhh there's that thick Southern accent." Apparently, "Y'all" is highly-concentrated.
One more story... My mother, originally from Nevada, was managing a band in Texas in the 80s. There were both men and women in the band. She had to take a phone call so as she left the room, she said, "You guys keep practicing." When she returned, the men were practicing and the women were sitting on the side watching. Of course, she should have said, "Y'all keep practicing."
I would add "You guys" as a substitute for "Y'all" in our area (Northern Nevada). And how about the use of "Yous" in the Bronx? Seems to serve the same purpose as "Y'all". I've even heard, "Yous guys," which I love. I'd use it, but I don't think I could pull it off.