Yes, you can trade Pokémon holding Mega Stones between Gen 7 games. I discovered it the lucky way, but it was worth it. Straight to the point:
I had my Charizard holding a Charizardite Y and the other guy asked to see my Pokémon via link trade. When I showed him my Charizard holding the Mega Stone he said he wanted it. I carefully chose what I wanted: He had a Marshadow! I didn't know it was possible to trade special Pokémon even via link trade, but IT IS. I accepted and voilá! I didn't know it was possible to trade both situations.
I tested it online. The Marshadow is legit. I don't have Marshadium-Z of course, but I have Marshadow, who cares? Just needed some hyper training.
Can you trade pokémon holding Mega Stones? Yes.
Can you trade special Pokémon like Marshadow, Arceus, Magearna? Yes.
Advance in the Battle Tree, guys. There are plenty of lazy trainers who want your Mega Stones for rare Pokémon they don't want/don't like.