The OS has no concept of an “fn” key. The keyboard never sends an fn keycode to the host machine. It’s a feature that’s entirely handled by the keyboard firmware itself. Your computer either receives an F2 key or a “brightness down” key, but it has no idea an fn key was involved in that one way or another.
So you could maybe modify your keymap to swap things out yourself. Intercept the “brightness down” keycode and manually map it to F2 or whatever. That’s the only in-software solution I can think of. That’s basically what the BIOS toggles do, as far as I know. Less than ideal to do yourself, though.