I don't have a solution, sorry.

But it got me interested, so I looked around and found how the front (white) LED can be manipulated:

echo [0-255] > /sys/devices/platform/i2c_omap.2/i2c-2/2-0032/leds/lp5521:channel0/brightness

And the screen brightness:

echo [0-255] > /sys/devices/omapdss/display0/backlight/display0/brightness