OK cool, at least you've made some progress. Just found this forum after searching about the brightness of the Kobo Glo. I was really disappointed because I fell for the marketing saying it could be read "even in pitch darkness". It's way too bright at minimum level for my eyes.
I guess if the ioctl doesn't work, we need to go lower and look at the device driver that ioctl interfaces with (if that's accessible, I've no idea as not tried to root mine yet).