I'm not sure if this is a solid argument when clearly there is major new content in ORAS that does not even exist in X and Y (like, I dunno, new mega evolutions). I'm pretty sure a patch is inevitable because Mega Stones are items, and Mega Evolution is not just a stat, appearance, and ability change (compare to Giratina, Rotom, etc).
I'm afraid Mega Evolutions
are just a stat, appearance and ability change, essentially. There's just a Mega Evolution animation involved, but it can easily be played without changing the model of the Mega Evolved Pokémon.
I'm a bit doubtful that you would even be able to battle Mega Sceptile, Swampert, or Diancie on ORAS using X and Y because X and Y will have no damn clue what item they're holding or how to interact with it, especially regarding moves and abilities that involve items, like Knock Off, Trick, Frisk, Magician, etc...
"Gourgeist frisked Sceptile and found its..."
See what RASFA said above (with terribly bad grammar, punctuation and spelling, but his point remains the same). In Gen. IV, Giratina received Griseous Orb in Platinum. It was a hold item, yet it didn't exist in D/P. The solution? Display the item as ??? when viewed on a D/P cart (such as before a trade). Knock Off, Trick and Frisk already existed back then. When you tried to trade a Giratina-O from Platinum to D/P, the Griseous Orb was automatically bagged. The Griseous Orb, just like Mega Stones, could not be Tricked, Knocked Off or otherwise removed during battle.
"Banette frisked Giratina-O and found its ???"
No problem. They could even just label the unknown items "Mega Stone", though I guess ??? would be most likely.
I'm 99% sure that Mega Stones would work the same way. To an X/Y cart, the Mega Evolution animation would play, but it would look as if the evolving Pokémon didn't change at all.
Start with the likeliness that a DLC patch for X and Y for compatibility for the new Megas (and even more specifically their items, the stones). Then factor in that GameFreak is likely aware that Groudon, Kyogre, and Rayquaza are the only box art legendaries that don't have signature moves. Also, GameFreak has CLEARLY taken a liking to adding signature moves, if you haven't noticed that almost all of the new Pokemon introduced this generation received one. (Although, they did Ho-Oh no favors at all...)
We have no precedence for adding new moves in the middle of a generation. We
have presedence for moves changing animation, and in some cases even stats, in the middle of a generation. We also know that GameFreak added a bunch of unused moves to X/Y, which would have been pointless if they had just planned to patch moves in later. In all previous generations, signature moves for event legendaries were added to the code of the "base games". I'm pretty sure this indicates that there won't be any new moves this generation either.
If I'm wrong, though, and we do see
entirely new moves, abilities or Pokémon species, I'll change my profile pic to a cod for a month, and personally apologize to you in my signature. I'm that certain.
Skore, I hope this answers your points as well.