I'll keep it simple.
In this question Palkeon asked if Pokémons holding a Everstone could Mega Evolve, which was answered (understandably) with a no, cause they need to hold a Mega Stone to access the metamorphosis.
But what about Rayquaza?
With the lack of a need to be holding the Mega Stone to proceed with the transformation due to his move, what would happen if , hypothetically, Ray were to hold a Everstone??
Let me know DataBase!