Rick Wong, who prefers to go by the name of Lick, has released his DS homebrew demo called Brightness Test. The demo (with src) shows how you can set the brightness in DS Lite using using libnds. Although this can be done using Moonshell and ALite, Lick reveals the code to set the brightness:

writePowerManagement((4), level); //level = {0-3}

The demo includes:

* Backlights on/off
* Lite brightness controlling (Phatty will use 1.)
* Read battery status.
* LED control.
* PersonalData reading

Please check the readme file for more info about this.