Depending on when Ash catches Sableye, it could just be 3 months before the X and Y saga begins. If it is indeed setting up a Sableye evolution, Ash's Unova team would be the last thing on the producers' minds. Aipom is a perfectly good comparison; a seemingly-superfluous old gen Pokemon caught...
Also, we've had several pieces of anime-related merchandise with Genesect in them.
No smoking gun, but it's pretty damned likely, even putting aside the fact that, were Genesect not debuting before the movie, we probably wouldn't need the movie Genesect to be shiny at all.
Little, if any, of the initial trailer actually made it into the finished movie in anything like its original form, from a Dark Ash confrontation to the design of the city to the legendary beast reveals.