Rick Wong, who prefers to go by the name of Lick, has released a demo of his homebrew app called DS Lite Brightness Test. The demo (with source) shows how you can set the brightness in DS Lite using libnds. Although this can be done using Moonshell and  ALite, Lick reveals the code to set the brightness level using arm7 code:

To set the brightness:
writePowerManagement((4), level); //level = {0-3}

To read the brightness:
readPowerManagement((4)) - 64; //returns {0-3}

• Backlights on/off
• Lite brightness controlling
  
• Read battery status
• LED control
• PersonalData reading

• Not tested on the Phatty, yet.
• PersonalData is UTF-16, which means the display of non-ASCII characters is incorrect.