Digging a bit I found that some power switch rules for gpio-keys are already present :
/usr/lib/udev/rules.d/70-power-switch.rules
Code
ACTION=="remove", GOTO="power_switch_end"
SUBSYSTEM=="input", KERNEL=="event*", SUBSYSTEMS=="acpi", TAG+="power-switch"
SUBSYSTEM=="input", KERNEL=="event*", KERNELS=="thinkpad_acpi", TAG+="power-switch"
SUBSYSTEM=="input", KERNEL=="event*", ATTRS{name}=="twl4030_pwrbutton", TAG+="power-switch"
SUBSYSTEM=="input", KERNEL=="event*", ATTRS{name}=="tps65217_pwr_but", TAG+="power-switch"
SUBSYSTEM=="input", KERNEL=="event*", ATTRS{name}=="* WMI hotkeys", TAG+="power-switch"
SUBSYSTEM=="input", KERNEL=="event*", \
SUBSYSTEMS=="platform", DRIVERS=="gpio-keys", ATTRS{keys}=="*,116|116,*|116|*,116,*", TAG+="power-switch"
LABEL="power_switch_end"yet, it is not working 
This is on a new install of LibreELEC 8.2.4 from scratch, maybe someone could point me to what I am missing...