In some way, we could find that out, without events etc. NOW. Just do it the same way, like they did with the strange door in R/S/E in Xeneroville. You can put that tree, who is in front of the room, with a computer-programme away, in the mysterious room was a battle, when i remember right. Nothing special, at all.
But for the secret room in Mr. Backlots' mansion, i have a idea. Whats about Phione or Manaphy? In his office, a diary is, where his favourite Pokémon, Manaphy, is shown. Probably, Nintendo will give sometime a event, where we could get somewhat like "Audience Card", what we have to show the maid, and then we could get the Manaphy, which is in a Pokeball on a desk.