On my Toshiba laptop I have two backlight devices in
intel_backlight. In Xorg, you could create a .conf file to choose which one the system should use. How do I do that in Wayland?
To clarify, you don't "do it in Wayland," since Wayland is just a protocol. It seems that configuring the backlight will need to be done based on either drm configuration and/or the particular display server config.
The details for Weston are in the
backlight_init function in libbacklight.c.