Hi,
had the same problems.
I just copied the file
/boot/overlays/rpi-backlight-overlay.dtb from a raspbian installation to
/boot/overlays/rpi-backlight-overlay.dtb in OSMC.
After a reboot you can control the backlight of the touchscreen.
sudo su
echo 1 > /sys/class/backlight/rpi_backlight/bl_power #turns the light off
echo 0 > /sys/class/backlight/rpi_backlight/bl_power #turns the light on
sudo su
echo 80 > /sys/class/backlight/rpi_backlight/brightness #adjusts the brightness. 255 is full brightness, 0 is off
Brightness control is only available if your touchscreen is version 1.1
Max