No, I've found cobalion and their friends before the league.
I don't want to spoil anymore but this is the cave with a 'karateka' design character who stop us and said the master come sometimes to train him here...
It's between the 'train road' and the dragon town...
I think kyurem is there because I don't see another cave. And It was the same with Virizion (a man still to block the road and said : I feel the presence of a powerful pokemon). So if this man block the cave it's probably because there's a powerful pokemon... like kyurem. (It couldn't be Cobalion, Virizion, Terrakion or Tornadus because I caught them so there are only two solutions).
This cave contains the most powerful pokemon. (pokemon with higher level in the game)
Or this cave contains a legendary pokemon.
I think about the second. The unova champion don't go there if this cave weren't an important place...
And you forgot that Reshiram could easily beat the little Zekrom
I remember during my first game on White and Black.
I had easily won my battle with Reshiram and It was hardest with Zekrom.
Nevertheless the configuration was the same. N had his dragon at the level 52 and I had mine at the level 50.