Since Pokémon can Mega Evolve during Contests, perhaps we can face off against Wallace in (high-level) Contests, and he can use his Milotic and Mega Evolve it--similar to how you could participate in high-level Contests alongside certain Gym Leaders in gen 4. Speaking of gen 4, if Jasmine makes a cameo appearance in ORAS (which is possible, because she wouldn't be too much younger in ORAS compared to DPP/HGSS), she could be an opponent in high-level Contests, just like in DPP, and perhaps she could show off a Mega Steelix. It seems like an odd choice, because it's a gen 2 Pokémon, and Onix can't even be caught in the original RSE, but Contests, especially post-game & National Dex, seem like a good way to introduce certain Megas to the player. It seems like they'll be using that method to introduce Mega Altaria, at least; that's what it seems like from the screenshot shown in CoroCoro.