The koko-aio docs describe it like this:
textures/side_shade-helper.png
Bezel frame inner area, the reflective part, is shaded by default so that
upper area is less bright and lower area is a bit brighter.
While the shader does not expose controls to change that, if you want,
you can edit this file to alter the shades.
The more a primary color is saturated, the more the shade will turn to dark.
So maybe it’s already working on top of your bezel wether custom or default (I didn’t confirm). I think rotation (90/270/180) works now by automatically rotating this side shade helper texture too, alongside bg and spot.