Okay, without a visual interface and the Ethernet, the only work-around I could think of could be a bit of a hassle.
It could be done by connecting a USB to the TTL serial cable on the UART0 connector. (You could refer to a pinout diagram of the Pi to know which is which.)
And, then type in these commands or instructions in your command line to get your Pi's IP address and connect to a WiFi network:
am startservice -n com.google.wifisetup/.WifiSetupService \
-a WifiSetupService.Connect \
-e ssid '<NETWORK_SSID>' -e passphrase '<NETWORK PASSPHRASE>'
Hehe, told you it's a hassle. :p