I've just done a clean installation of Devuan Jessie and then migrated
to Devuan Ascii, both with XFCE as the desktop environment. There,
/usr/bin/xfce4-keyboard-settings allows setting keyboard shortcuts for
most keys:

Settings Manager -> Keyboard -> Application Shortcuts

However, something is off with XF86MonBrightnessDown and
XF86MonBrightnessUp. No matter what I set in xfce4-keyboard-settings,
pressing either key won't trigger an action. Other keys can be assigned
actions, such as fn+F7 or fn+F8, but not XF86MonBrightnessDown or
XF86MonBrightnessUp

If it is relevant, I installed from
devuan_jessie_1.0.0_amd64_uefi_desktop-live.iso and not
devuan_jessie_1.0.0_amd64_desktop-live.iso before migrating to Ascii.

What should I look at to get those two keys to trigger scripts and programs?

/Lars