Thanks, Georgy!
On the hardware side, I am aiming for a small footprint and low power consumption. To get actual use, this display device will need to be "always on", so power consumption is a key requirement. A simple all-in-one package would also be good, ideally without a power switch (or even multiple switches) which one can inadvertently switch off.
A re-purposed Kindle reader is a tempting option, since it would come with a low-power e-ink display and a nice compact package. I need to get a better idea which jailbreak works with which version -- the information I have found so far is a bit messy, and it's hard to figure out what is still current.
Software-wise I am hoping to find an open-source client for Google Calendar, or for some other existing web-hosted calendar. That would take care of the backend, and would allow me (or other relatives) to update the calendar content remotely, with a decent user interface.
To my surprise I have not found such an open-suorce Google calendar client yet. Is Google not providing a suitable API since they want users to go through the Google website? Am I using the wrong search terms? Is there another web-based calendar which supports this better?