I haven't been able to test this yet, but I found this answer from
https://discussions.apple.com/message/16567046#16567046 :
"1. Connect to what you need to over WiFi (I did it for iPad 2 AirPlay mirroring to Apple TV).
2. Go to iPad's WiFi settings and tap on the ">" button to the right of the Router's name (SSID) to get to detailed settings.
3. Remember IP Address and Mask for DHCP tab, then tap on the Static tab and enter the same data in two 1st lines.
Now connect over 3G - all is working.
Explanation:
All WiFi settings are neded in order for Apple devices to see each other over WiFi and initiate connection. Then only ip is needed to continue exchanging the data. With static configuration not having router/DNS info, iPad can't use WiFi for internet, so it goes over 3G."
As soon as I can confirm, I'll let you guys know. Unless someone can do it sooner!