I'm gonna give my opinion of megas to help you out. I personally think they're kinda cool and a great edition to the Pokemon family. But I don't think, if you use them for the roleplay, that they should be activated simply through a piece of jewelry and a shiny rock. I think, again if you end up using them, that they should only be obtained after the person and Pokemon have some sort of unbreakable bond. And because of our characters are at war and are fighting, I think it should also effect the person in some way. Like they strong bond makes them both stronger for a period of time, but maybe tires out the person if they use it too long. Since normal people, usually, can't use Pokemon moves.
Just an idea and an opinion. Of course it is up to the GM.