I made a modification of FIX94's arm9select that turns the backlight on for default.bin if it's a cold boot (and if default_bl exists), but doesn't if it's an MCU reboot (meaning that you used something like Quick Reboot to reboot the 3DS, or exited a GBA game/DS game). That way you can still have the splash on the initial powerup, but the screen stays black and off like a stock boot after that, as long as you're using FIX94's arm9loaderhax fork as well. If anyone's interested in it, I can post it here.