How do I tell an iOS device there's no internet connection on the Wifi?

I've setup a WiFi network to allow iOS devices (such as my iPad) to connect to other devices on the local network, but I want to be able to use the 3G functionality on my iPad at the same time to connect to the internet. How can I setup the WiFi so that the iOS device doesn't think there's an internet connection through it?

I've tried setting the DHCP Gateway to 0.0.0.0 but this just seems to confuse the iPad - the spinning circle next to the Wifi network never disappears even though it has an IP address from the DHCP server.

Update: I was able to set the DHCP Gateway to 0 and the Router entry now appears as blank, but the spinning circle still doesn't disappear.

Update: Setting the DHCP Gateway to 0 or 0.0.0.0 works fine for Windows, Android and Windows Mobile so I'm beginning to think this is an iOS bug. I've posted a question on the Apple iOS Dev forum here.

You should define a STATIC address for the WiFi/LAN. (define an correct netmask, and dont enter any router/gateway address ). (see the static tab in the @Michiel's screenshot.)

When you done, you can access the computers on WiFi network and you will access the rest of the Internet via 3G.

This is what you looking for?

This appears to be fixed in iOS7 - I set the DHCP Gateway to 0.0.0.0 and the iPad connects with a tickbox rather than leaving the spinning circle.