I wrote the GalliumOS ISO image onto a USB stick and tried to boot it. It said that the USB stick did not contain a valid recovery image.
Recovery image? Maybe I should try to get ChromeOS working on it first before trying to boot another OS.
A search found a site holding images for many ChromeOS devices. I took the CN60 one. It's about 500 MB zipped and about 1.5 GB unzipped.
Wrote the ChromeOS recovery image on the USB stick and at the "scary prompt" typed Ctrl-U to boot from USB.
It took many minutes to restore ChromeOS. I power cycled it and at the "scary prompt" let it boot from SSD. I could also have typed Ctrl-D. It booted up to the ChromeOS splash screen.
So maybe I have a working ChromeOS device now?