Alright I know how to get in to the fancy ass boutique.

It wants to think you're spending a lot of time in lumoise city, so just go near one of the cabs, save, and quit.

Reboot the 3ds and set the clock forward a day.

you save near one of the cabs so you can quickly move to the boutique right after loading it.

I'm not sure why this is important, but it did make a difference for me.

Make sure you move fast after loading the game


It should work fine.
I did this, and only this, on my Y version after reading about it working on the serebii forums.

I did nothing else between being denied and accepted