I tried something similar, getting the key event to some MCE-“known” button through udev hwdb:
/etc/udev/hwdb.d/70-xperia10iii-assistant-button.hwdb
# - Generic input devices match:
#
# E: MODALIAS=input:b0019v0001p0001e0100-e0,1,k72,1C9,ramlsfw
evdev:input:b0019v0001p0001e0100*
KEYBOARD_KEY_01c9=controlpanel
# KEYBOARD_KEY_01c9=camera
and then in /etc/mce/99-xperia10iii-assistant-button.ini
[EVDEV_TYPE]
evdev:input:b0019v0001p0001e0100=INPUT;KEYBOARD;MOUSE;DOUBLE_TAP;ACTIVITY
[EVDEV]
# see https://github.com/sailfishos/mce/blob/master/evdev.inc#L607
# and https://github.com/sailfishos/mce/blob/master/evdev.inc#L29
#KEY_01c9=SW_KEYPAD_SLIDE
#KEY_01c9=SW_POWER
#KEY_01c9=SW_RFKILL_ALL
#KEY_01c9=KEY_RFKILL
#KEY_01c9=KEY_MICMUTE
#KEY_CONTROLPANEL=SW_RFKILL_ALL
#KEY_CONTROLPANEL=KEY_RFKILL
KEY_CONTROLPANEL=KEY_MICMUTE
But this also does nothing