You can use custom script like:
state=$(defaults read "Apple Global Domain" "com.apple.keyboard.fnState")
if ((state)); then
echo "true"
defaults write "Apple Global Domain" "com.apple.keyboard.fnState" "0" ## Brightness/Media
else
echo "false"
defaults write "Apple Global Domain" "com.apple.keyboard.fnState" "1" ## F1 F2 etc
fi
defaults read "Apple Global Domain" "com.apple.keyboard.fnState"