Configure power button to shutdown on Debian/PPC

I use Debian/PPC on an old Mac Mini G4, it currently serve as a DLNA server (UPnP), no mouse or keyboard plugged.

I would like that my power button also serves to shutdown the box. Currently it does nothing, on recent x86 I would have used ACPI as described here.

However ACPI does not seems to be available from my G4 box, only pbbuttonsd is available, see link.

I could not find whether or not any event (APM type?) is sent when pressing the power button. I know that I can hold the power button for 4s then the machine halt, but I would prefer a clean shutdown. As a last resort I could plug in a keyboard but that I am looking for a solution without mouse or keyboard.

How would one do that ?