So in a rare case of completionism, I've managed to build a viable 'finished' article of a printer connected to Slack.

Its rough around the edges - the power supply is pretty rubbish (USB to the RPi and a separate wall-wart for the printer itself), the external wiring is a bit messy (three cables rather than two) and the code is a little unruly but it works.

Next things to solve;

1. Power supply - Can I run the Pi and Printer from one source?
2. Images/Multi-Media - How can I print image embeds or avatars as part of the output