I generally hear y'alls's used as the possessive form, but I have also heard yourn. Since y'all is a colloquial pronoun, its possessive form is basically liberated from prescriptive linguistics which would probably say that y'all is a contraction for you all and therefore must be possessivized as of all of you.
Do you know if there is a "standard" way to make y'all possessive, insofar as y'all usage can be considered standard?
you allso the possesive would be your all. Though my cousins from Tennessee say y'all's. To me rhis is kind of like asking the the plural form of s'not (is not). There is no correct way to say something improperly. – Chad Jul 21 '11 at 21:02