The workaround for this is to push
Gapps for Android 5.1 x86 LeanBack: gapps-lpmr1-x86-leanback-20150524-signed.zip .onto the AOSP fugu build.
When you reach the WiFi password screen, the prompt says "Use the keyboard on your mobile device." English keyboard is not included, although Korean keyboard is in gapps, so select at start Korean language and go through the setup steps (the Korean keyboard supports latin characters).