For (my) future reference: Whether or not my keyboard works in a (wlroots) Wayland compositor depends on whether that compositor uses wlroots’s keyboard groups: https://github.com/swaywm/wlroots/pull/1771

Interestingly, this isn’t such a crazy rare scenario after all, see this comment regarding XF86MonBrightness*: https://github.com/swaywm/wlroots/pull/1771#issuecomment-515507790