To be honest I'm not entirely sure how to do this with a router, I've only ever done it with my wireless card in my PC.
It's the same concept as HomePass. You set up a wireless access point with the same name as the official Nintendo Zone access points, and use mac spoofing programs to spoof your mac address. Your 3DS will be tricked into thinking they are connected to a Nintendo Zone.
You can trick your 3DS into thinking it connected to a Nintendo Zone simply by changing your AP's name to attwifi or NZ@McD1, but in order to be able to download Hoopa you'd probably have to have a certain mac address so your 3DS thinks it's connected to a McDonalds access point, and not just any Nintendo Zone.
This page has a list of mac addresses for various Nintendo Zone access points, so it should be possible to spoof your router or your wireless card as having that mac address, then use the right SSID, and start up your router or wireless access point to download Hoopa as if you were at McDonalds