I didnt find a solution and I doubt there is one. I believe the Fn key is a hard-key, I've had cases where the whole notebook crashed but the Fn key continued to toggle.
I recently bought a $19 Logitech multimedia keyboard for my desktop which has a similar Fn key, and was pleasantly surprised when I installed the drivers and found a setting to disable the Fn key toggle. (So F1.F12 keys work as F1..F12 and you key multimedia functions only when Fn is held down.) I wish the Surface Book had the same feature.
I recently bought a $19 Logitech multimedia keyboard for my desktop which has a similar Fn key, and was pleasantly surprised when I installed the drivers and found a setting to disable the Fn key toggle. (So F1.F12 keys work as F1..F12 and you key multimedia functions only when Fn is held down.) I wish the Surface Book had the same feature.