I think y'all is so informal that it's almost irrelevant whether it's grammatically correct or not. Spoken English, especially slang, doesn't follow the same rules. And wouldn't y'all only be used in situations that are so informal that nobody cares about correct grammar, anyway, and not for formal writing or professional speaking?

"I seen," on the other hand, makes me want to chew glass. That seems outright incorrect, rather than just a colloquialism. But it might be possible to argue otherwise, I suppose. After all, what is "correctness," anyway? We use language with particular audiences, for particular purposes. Form is important mostly because how we use language affects how our message is heard and, yes, how we are viewed and judged. In most professional situations, correct grammar is very important, and it's essential in publishing. "I seen" would never fly there, but y'all might, if it were used informally or in a tongue-in-cheek way.