I haven't seen this talked about much, but they confirmed that ORAS megas will be in PLZA

Here they show off mega altaria, and right before this they show mega sableye. I don't know if every ORAS mega is going to make it in (like I'm sure fitting mega Rayquaza and the lati twins would probably be difficult) but it's good to know at least some ORAS megas will make it in