I can't see this being particularly likely, as it would mean too great a proliferation, unless they were to be sold at inordinate prices as a currency sink. There's just too much cash in-game. Those mega stones that were obtained for free from loot balls were never intended to be available from such a source.
Not only that, but aside from legends and pseudo legends, I don't think there are any Pokemon capable of mega evolution that can't be found in the wild, now that many previously box-exclusive Pokemon can be caught in the safari zone.