It seems LibreElec is lacking support for Apple MagicTrackpad (the external bluetooth device, first generation; maybe also the current one). The bluetooth pairing of the device seems to work fine, but no /dev/input/mouse device is created. It seems LibreElec is lacking a hid module `hid-magicmouse`. On my Retropie I see:
/lib/modules/4.9.35+/kernel/drivers/hid/hid-magicmouse.ko
While on LibreElec there is no such module:
I guess it might just a matter of enabling this module in the kernel configuration...
This is with LibreElec 4.2