I am using LibreELEC on a RPi3 with this remote: VRC-1100 . It seems like the power button is not programmable. I believe the underlying OS intercepts it before Kodi. Is there a way to tell LibreELEC to ignore that button?
The wiki (for Ubuntu) says:
Power button is captured through a different interface. Open dconf-editor application. Browse down to org.gnome.settings-daemon.plugins.power by clicking on the black triangles next to each path element. Locate button-suspend and change it's setting to Nothing. NOTE: Although, this prevents the computer from shutting down, the key still doesn't make it into Kodi. This needs work. Holding Alt on the keyboard and pressing Power button on the remote does send "sleep" command to Kodi.