In terms of BW2, I asked this twice and was told they weren't allowed twice. However, I never thought to ask about the Dream World changes. They might actually be allowed. I don't think there are too many unreleased Dream World abilities that would really shake up the metagame but an official Custap Berry release would likely change things. Hopefully nothing silly like a legal Soul Dew happens.
I would suggest everyone learn about the relevant BW2 and Dream World changes just in case. I may be paranoid, but I'm not fully convinced it's an issue they had thought through 100%. If you have the time and resources, you could also take the extreme measure of making and bringing a team that takes advantage of BW2 on the chance that the changes are considered legal.
-Players may use Pokémon with Hidden Abilities that have been officially released via Pokémon Black Version and Pokémon White Version, the Pokémon Global Link, or an official event or promotion.
from the rules. so hidden abilities from black and white 2 will not be legal.