It's become obvious that a coprocessor is needed. Some of the functions needed:
- Handle booting up and safe shutdown of the RPi (read battery via ADC)
- Handle button polling and updating RPi
- Display splash screen while Pi is booting (a linux boot screen breaks the illusion)
- Play startup sound via DAC to speaker
The ATMEGA32U2 or 16U2 is an obvious choice as it has a build in HID module that can communicate over USB.