Everything looks good! All that's missing is a hole for the sliding power switch on the back cover. I wired the low bat pin on the Adafriut PowerBoost to the Teensy. When the battery is running low the teensy can send a message over the serial USB to the RPi3 and pop up a notification to the user. Software and code are pending and will be added to github soon.
Keyboard behaves as a standard USB keyboard as expected using PJRC's excellent USB Keyboard and Mouse library: https://www.pjrc.com/teensy/td_keyboard.html There's some bonus functionality for using the D-pad to move the mouse cursor around. That turned out to be useful for the non-touch screen. By the way this really needs a touch screen, I've repeatedly tapped on the screen expecting something to happen despite knowing it doesn't work.