I do live, however nominally, in the south, and that's definitely my intended response to Antoninus on most things: "Bless Your Heart". ;-D
Sometimes there's nothing else to say except "Bless your heart."
However, I've recently noticed, "Hey, I'm just sayin ..." being used in a similar context. It seems to mean, "You want me to shut up, but I won't, and you can't hit me, because I'm too far away, or people are watching, heeheehee."