I am not sure how Mate power manager does map the /dev/input/eventX device for the hall sensor (you can use evtest to see what events the hall sensor sends), it just works there. But it should be possible to map that at a lower level than the Desktop environment, maybe check upower or even udev rules... also Armbian is using Xfce by default for their Desktop images and they have a beta one for the Pinebook, so maybe look at that too...
Come have a chat in the Pine IRC channel >>