I have an old Devastator CM Storm keyboard.
The backlight is toggled with Scroll Lock, with the backlight on while scroll lock is active. Unfortunately, PSO2 seems hard coded to do some direct chat interface with Scroll Lock. Or at least I didn't find an option to change the keybinding in settings. And I can't read the keyboard when the backlight is off. Seems I can't play keyboard+mouse as I am stuck in the direct chat interface via Scroll Lock.
Not to be forwardly rude -- but please spare me comments like "buy a new keyboard" . I've been gaming for decades, and this is the first and only time that Scroll Lock has been an issue. And no, I am not opening my keyboard to do a hardware hack.
Edit: Gamepad seems to be working, and it isn't terrible, either. Still, even though this is a niche case, it would be good if there was some workaround for keyboards like mine.