Faldage, I've been grinning ever since I read your post!
Wordwind gave a wonderful explanation of how it likely evolved. But I doubt there's a "correct" rule for spelling this, because as was pointed out, it is a colloquialism. Until I started putting it here, I don't think I'd ever seen it written, only heard it spoken. And I put your-all's, because your-alls' just doesn't taste right. So, if this is the only place you've seen it written, feel free to correct me right and left--I'm quite honored, to tell you the truth! I suspect the s got migrated down to the end for the same reason words such as spoonfuls and handfuls developed--it just comes easier off the tongue, to speakers of a language where the overwhelming majority of pluralizations are at the end of a word.