At the moment update of netboot systems is unconditionally disabled:
LibreELEC.tv/init at master · LibreELEC/LibreELEC.tv · GitHub
LibreELEC.tv/init at master · LibreELEC/LibreELEC.tv · GitHub
LibreELEC.tv/init at master · LibreELEC/LibreELEC.tv · GitHub
I don't see why it wouldn't work for r/w share. Settings restore works just fine, and system update is no different.
If changing this behaviour is a no-go, a new kernel init argument overriding this would be appreciated.