I hate to be the bearer of bad news but, what KoN was getting at was that it is impossible for MEGA heracross to hold a king's rock or a razor fang due to the sheer fact that it is a mega evolution only ability. In order to mega evolve and get this ability heracross must hold the heracronite. Therefore he is unable to hold a king's rock or razor fang, and it is impossible to trick/switcheroo/whatever a king's rock or a razor fang to him because it is impossible to remove a mega stone from a mega Pokemon. I really hate to herald bad news but I had to.
(Sorry, don't come after me with a Frankenstein-esque mob with pitchforks and torches. D: )