what about trying to get into the BIOS via Esc, F10 or Del (or whatever it is for your laptop)?
the hope is the laptop won't honour your brightness level when entering BIOS and I've seen some BIOS versions where you can change screen brightness from there.
and just to clarify, you have tried F2 or F3 with the Fn key, what about without it?
as some laptops have the Fn key preset to active or not, meaning to use the F key special action, you only need to hit that F key without Fn.