It appears that "spoofing" a Nintendo Zone works perfectly well for cross-country/continent Streetpass relaying.
- Change your wifi router/stick/phone MAC address to a shared one.
- Change your router/stick/phone SSID to attwifi (or any other "good" Nintendo Zone ID)
- Close your 3DS in sleep mode.
Now, I've only gotten ONE Streetpass (out of three attempts), but others have gotten my streetpass data. When I rebooted the router after changing the MAC and SSID, it worked better. I'm suspicious there is still a time delay between Relay Streetpass attempts, though, as I can't connect to a point that others are.
This is culled from a thread over on GBATemp that someone else linked to here on NeoGAF.
If anyone wants to try this, we just need to pick a MAC address for the NeoGaf Virtual Relay...