I was just looking in the WebOS Internals wiki to see if I could find something interesting.
I found this page: Controlling LEDs from the Shell
At the end, he says that he will maybe make a shell script to manually switch the state of the LEDs. But he didn't make one.

So I made it It's a 1-minute-script, follow the rules or it will eat your cat and sell your house.

Put it wherever you want, launch with sh:
# sh leds.sh center 0
will turn off the center LED
# sh leds.sh left 50
will turn the left LED half-power.

SYNOPSIS: sh leds.sh [left|center|right] [0,100]

Have fun !

The script: http://clouds.dyndns-server.com/blogsimon/leds.sh