After gen 6's first battle competition was announced yesterday, the official Korean site may have accidentally leaked the ruleset. It is as follows:
First, the battles are to be Double Battles and you can use any Pokémon in the National Pokédex, except Mewtwo, Xerneas, Yveltal or Zygarde, and you cannot use transferred Pokémon from previosu generations. All Pokémon will be made to be Level 30 in the competition. In addition to that, there's a new regulation included. In this competition, you can only have 20 battles per day, but if you participate in less than 20 in one day, you can make it up in another so if you make 15 battles in Day 1, then you can do 25 battles in Day 2. After the competition, there will be a survey on how it was run, running from February 28th to March 7th.
The key point here is that you can't use transferred Pokémon. Every Pokémon that's typically banned in tournaments HAS to be transferred, aside from the four mentioned.
To one notable exception: Phione.
Sure, you have to transfer over a Manaphy or a Phione, but then you can start breeding them and make Phiones to your heart's content. And as far as I can see, nothing in the rules prevents that. It's not Mewtwo, Xerneas, Yveltal or Zygarde, and it's the offspring of a transferred Pokémon, it's not a transferred Pokémon itself. Yay for loopholes! Now, the million dollar question: can you transfer a Manaphy egg into Pokémon Bank, then X or Y, effectively making it a Kalos-native Manaphy? It'd be so hilarious if that was the case and everyone in charge just missed that.
My best guess as to why they're so insistent on having Pokémon participating in tournaments be born or caught in Kalos is that to the best of our knowledge, there's no Pokésav or anything like that for gen 6 games, so any Pokémon that's from Kalos is pretty much guaranteed to be legitimate. Unfortunately, it has the side effect of completely preventing the use of any legendaries from gens 2 through 5, as well as Mew and... Unown. Yep, Unown is totally incapable of breeding, so there can be no such thing as a Kalos-native Unown. Some tutor-only moves are also a no-no.
Edit: Turns out the banlist was incomplete. The typical banlist is what's enforced here, so no Phione. Again.