I did a little experiment with Samsung Dex to see if the Android-only environment could work. So far - I found it way less distracting not having all my current tabs/finder windows open. I could definitely see it as a nice "writing appliance" that I could put in a quiet corner for the end of the day.
I'm using termux, so I ssh in to a machine where my drafts are (although I have SyncThing installed and can theoretically use that, I haven't bothered). The ThinkSmart can lock itself in the usual ways.
What's funny is that what you did with a Pi Zero and Plan 9 is basically my ideal when put in conjunction with one of Lenovo's USB ThinkPad keyboards. The TrackPoints on those have three mouse buttons and work well, and the keyboard is sleek enough.
It is a standard LCD display with amazing contrast (black is just... black, and I use a white-on-black full screen terminal), although I wouldn't want to get into a debate regarding safety here (let's just say I don't subscribe to pseudo-science).