When my Wii U freezes and I power off the console, the gamepad screen looks like this briefly until having the normal lost connection to the Wii U console screen. Afterwards it
Portable RPi Console Built into a Wii U Pro Controller - Hackster.io
Wii u, Wii, Arduino
Wiimote Controller Configuration for Raspberry Pi 2/3 : 13 Steps (with Pictures) - Instructables