How to connect 3rd party tcp/ip device to Nexus7
The obvious solution would be to use a device that has both ethernet and Wi-Fi as a router or a bridge. If it joins the same Wi-Fi network that the Nexus 7 is on, then the Nexus 7 will be able to make a connection through the router/bridge to your mystery device, while also talking to the Internet through the same Wi-Fi network. Alternatively, if the Nexus 7 is on 3G, switch on its Wi-Fi hotspot and have the router/bridge connect to that.

I don't know if any commercially available special-purpose Wi-Fi routers have the ability to act as a client rather than an AP, but this would be easy to rig up with a Raspberry Pi or similar small Linux machine.