In the meantime I was also searching the Internet to find what hardware was inside. The best page I found was in the Kodi wiki (archive.org capture, Kodi has updated their Wiki) although this Chromium page has some useful info too. It's from 2014 and has at least a Celeron processor and 2GB RAM, so less powerful than my current NUC. Might make a good entertainment box or auxiliary PC.

First thing I learnt was that I should put it into developer mode, using a paper clip and the procedure in the Chromium.org page.

Further searching found that there is a Linux distro dedicated to ChromeOS devices, GalliumOS. So I downloaded the ISO image for Haswell CPUs.