This is how I was able to hack around settings for my elecom huge trackball on plasma+wayland (for comparison: similar xorg setup: https://gist.github.com/WaffleLapkin/8b0b739b705e91a0dd9babccd0907267).
/etc/udev/hwdb.d/70-elecom-huge-trackball-remap.hwdb
:
# Rebinds keys on elecom huge trackball
#
# tip: You can use `sudo evtest` to get the key values and stuff.
#
# 90001 = L
# 90002 = R