After looking in the source I realised that some system settings are also prefixed with system. , and I didn’t notice that to start with. I’ve correct that and the config is created. So it’s:
snap get system system.power-key-action
Feels a little awkward, but once you know it it’s fine.
Something else I discovered, as I was setting this as a default via the gadget initially, is will allow you to set arbitrary settings that don’t exist. Whereas if you try that via snap set you get an unsupported system option error.
it might be a bug, but note that if this bug would be fixed, the first boot seeding would fail once you set a wrong option, so your system would not be able to finish booting and you might have have a very hard time to find out why exactly the boot failed then … it is probably the lesser of two evils to allow setting random unused options…
I would say file a bug for this in the snapd project on LP, as @ogra said we may not want to fix it, but perhaps we could add a warning to snap warnings or something less fatal than blocking seeding entirely.