Like, what happens if I trade a gallade holding Galladite (a new thing not in XY) from ORAS to Xy? Does it dissapear? Will it work? I have no idea and I'm not trying it until I know it will work.
Short answer is no, you can't do that. ORAS Megas cannot be obtained in X and Y - if you try to do it, the game will stop you.
It is impossible to do that as the coding of the new megas is not in X/Y, so the game will stop you.
It will not allow you to trade any new items, cosplay Pikachu, or Rayquaza with the move dragon ascent.