Looking through the source code is what I should have done first!
~ $ meshtastic --ch-set simple1
Connected to radio
Writing modified channels to device
~ $ meshtastic -- info
[...]
Channels:
Primary psk=simple1 {"modemConfig":.....}
simple0 (default) - simple254 are allowed variables.
random is still giving me the error though, so I believe there may be a bug which I’ve filed.