It seems like it's impossible no matter what you order or how many times you order, I doubt Game Freak would just put an area in the game just to make it impossible to access, but I don't know, maybe I've missed something, is it possible at all? What about if the game's story is finished?
You can "technically" get up there, but you are unable to explore it. Once you beat the elite four and you order a meal there, the man at the counter exclaims that you're a "VIP." Then you just get a clip of yourself eating on the upper level. Other than that, you are unable to reach that area.