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If we could, wouldn't the forms have been found somehow in the coding of X/Y or ORAS? We learned about Hoopa-Unbound and the two extra Vivillon forms that way.
That'd definitely be a neat surprise if after all this time, there was a way to get it and we just didn't know about it :o But like Nah said, someone would have found it by now. The other Zygarde formes aren't even in the game's code (its signature moves are though, Thousand Arrows and Thousand Waves).
As Sopheria said, Thousand Waves and Thousand Arrows exist in the coding but alternate Zygarde forms don't. We have had items that existed in game coding and never saw the light of day, like the God Stone in B/W, and also D/P/Pt's Azure Flute which was planned to be an Arceus event that ended up getting scrapped. The Thousand moves seem like they might suffer the same fate.
We'll see if they plan to add this into further gens, to allow for trades back, or perhaps just various attacks. That said, though, trainers generally wouldn't want Zygarde to be much less than its orthodox form - and presumably any Zygarde knew how to do this, and as a result if it obeyed the trainer would usually do so - and in that sense it might not be entirely worthwhile.