The current code is very rough, but I added support for held keys (for volume up & down). Just for reference, the alpha version is in attachment. Keyboard support requires root access, so I modified one line in phoniebox-gpio-buttons.service:
It would be nice to also enable Chromecast streaming via catt. I'll see if I can include that as well.
Once I'm back from vacation and I have the final small bits of kit, I'll post a detailed how-to with a more robust script.