Sorry to say, it's confirmed to be a April Fool's prank. This is from Zone's tumblr.
"For those of you who have not yet realized it, ZONE-tan as DLC was an April Fool.
I, and a lot of the folks at Lab Zero have been asked recently if ZONE-tan would ever become a fully fledged DLC character and our response has always been that fan-characters are not something that would be considered for the game.
Even though I?m doing contract animation work for Lab Zero, Skullgirls is Mike, Alex and the rest of the core team?s game, not mine. ZONE-tan has no real place in the Skullgirls universe other than as a BG character with every other Patron Saint?s OCs, and is no more important than any of the rest of them. Alex and Mike already have over 30 genuine Skullgirls characters ready to develop as full-fledged DLC or possibly in a Skullgirls sequel, so there is absolutely no need to get fans to start creating characters for them.
Lab Zero have already been extremely generous, adding an homage to my Filia flash into the 1.01 patch, saying that they may consider a ZONE-tan Squigly palette and allowing me to have ZONE-tan as a background character in one of the new stages for the Patron Saints? OCs. I don?t want to attempt to shoehorn any of my content into the game where it would have the potential to taint the wonderful universe that Alex has developed, I already feel that they have done way more than enough.
I would rather see any of the other characters that Alex has designed appear in the game before ZONE-tan. It would be extremely rude of me, after all the courtesy they have shown me, to attempt to push for them to consider anything like this, and I honestly don?t want to see it happen. There is no need to pester Lab Zero about getting her in, or trying to start some campaign to see it happen, it?s just not going to happen. No one has forced my hand, this is just what I feel is right.
I hope the April Fool at least gave people something to have fun with. It?s as close as she?s going to get to being in the game, and that?s the way it should definitely stay."