There is some confusion about whether Ditto can hold the Mewnium-Z (or any exclusive Z-crystal, for that matter). Counter to what the game makes it seem like, you are 100% able to give Pokémon Z-crystals that they are incompatible with. Under normal circumstances, this just wouldn't do anything. For instance, here is my entire party holding Mewnium-Z.
Although a bit counterintuitive, you are able to do this, even in the face of the "Incompatible" warning:
Therefore, a Ditto holding Mewnium-Z can use Genesis Supernova if it is transformed into a Mew that knows Psychic. Similarly, Ditto (or any other Pokémon with the ability to Transform) can use an exclusive Z-move of any Pokémon it is transformed into, as long as it is holding the appropriate Z-Crystal and the Pokémon it transformed into knows the correct move.
I found a video of someone proving this by having their Ditto transform into a Tapu Fini while holding Tapunium-Z and use Guardian of Alola.
Therefore, for a transformed Pokémon to use Genesis Supernova, they'd just need to be holding a Mewnium-Z and transform into a Mew that knows Psychic.