How can I configure my keyboard to be backlit at the login screen?

It appears that the problem is related to FileVault. When I turned FileVault off and rebooted in safe mode the problem went away. I could change the brightness of the screen and the keyboard at the login screen. Volume controls worked too. Now that I've turned FileVault back on the problem is back.

Have you tried turning up the keyboard backlight brightness at the login screen, logging in and restarting to see if it will keep your setting? Typically, the setting for the backlit keyboard is stored in the PRAM. Once setting the brightness to your desired level, it should retain the setting after a restart providing you don't reset the PRAM. Give it a shot.