Is this installed? It’s stated this driver should add backlight support into sysfs so UPower should handle it.
UPD. I read your messages again and see that it’s installed. Well, I think it’s up to driver to implement led_classdev interface. There’s a PR for it but it’s quite old and it seems no one interested to merge/update it.
Try to open a new issue about it there maybe so they will take a look into it again?