So functionally, Z-moves are essentially "Mega moves". But read this...
Key Items for Z-Moves
The Z-Ring fits on a Trainer’s arm, and Z-Crystals are set into it. If a Pokémon holds the same variety of Z-Crystal, the two will be able to resonate with one another. Crystals that correspond to each Pokémon type have been found in the Alola region.
According to the in-universe flavor, you need to keep a crystal in your bracelet, AND have your Pokemon hold an identical crystal, so they can match up and do the move. But if you remember from gen 6, a "Key Stone" was a thing that the characters wore to match up with the Pokemon's held Mega Stone. The player characters wore these in a bracelet as well.
Is it possible that you would have to choose between having a Mega or a Z-move on your team, because your character can only store 1 item in their bracelet at once? Or will they throw out the flavor logic and let you do both (maybe like the XY NPCs who stored their keystones in other jewelry not on their arms)?