I've had a few questions about whether this project could be ported to an ESP8266. It turns out the answer is yes:
This port is at a very early stage, so I'm not releasing the code just yet. It runs, but loading and saving of settings (and tapes) doesn't work and it makes no use of the WiFi capabilities of the ESP8266.
I'm using an Adafruit Feather Huzzah here but it should work on other ESP8266 boards like a Wemos D1 mini. The screen is the same as the Teensy version and I'm using an MCP23008 I/O expander for the buttons due to a shortage of GPIOs. I might be able to get around that with a bit more work though.