I'm not sure about openwrt, but I use Mikrotik/Routerboard products, and I can use either protocol. I connect the Arduino to the usb port on the Routerboard (gets power there too), and the ethernet shield to one of the ethernet ports. I can ssh into the router and connect to the Arduino usb like a serial monitor, or use the ip address with TCP/IP as either a server or client.
Maybe openwrt has that usb feature also.
Actually,i just need to do some programming on the router to let it do more controls.I wonder which system does the routerboard runs and i wonder if you can let the router board deal with the things itself.