Yeah, honestly I don't think anyone should get their hopes up too much about that one. Metallica aren't known for "hanging out" if you know what I mean, especially at something as big and crowded as SXSW. They'll be flying in on the day they appear, driven to the venue, and then leave afterwards.
THE ANSWER got added to the Ac/Dc tour having nothing to do with what label they're on, from what I understand. It's all about who your manager is, who your booking agent is, what connections they have, etc.
Sorry to be the party pooper but it's the truth.