DSLite Brightness: Found the missing bit!
I found the bit in the UserSettings that stores the Nintendo DS Lite boot-up brightnesslevel. By reading/writing to this address, we can make the Lite boot-up with the desired brightnesslevel. Unless I’m slow, this hasn’t been done before.
Anyway, the bits are at the Language and Flags variable in the UserSettings. Bit 4 and Bit 5 will have to value of 0-3 (2 bits) indicating which brightnesslevel is stored.
(For read-only, libnds provides PersonalData->RESERVED3.)