I know you can't trade the new ORAS Mega Stones to XY but can all other held items be traded?
The only things that can't be traded between ORAS and XY are:
Groudon or Smeargle with Precipice Blades
Kyogre or Smeargle with Origin Pulse
Rayquaza or Smeargle with Dragon Ascent
Pokemon holding items available only in Omega Ruby and Alpha Sapphire, including the items for the new Mega Evolutions / Primal Reversions
All other Pokemon/Items can be traded between the games.
To put it bluntly,
Items cannot be tranferred between Gen VI games via Pokébank. However, all items (including Mega Stones) form XY can be found in OR/AS. For more information, use this link: http://www.serebii.net/omegarubyalphasapphire/items.shtml
We can only hope that Game Freak will make the two Gen VI games fully compatible, like they did with HG/SS and Platinum in Gen IV. However, this is unlikely :(