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.