I think the invert option is simply impossible. I think Dragoro and some others tried to explain, and I don't want to be part of the Mouse Nazis, but let me try to explain?
Normal mouse style (FPS, Windows):
This is currently used in Freelancer. We've all heard that FL is unique in its controls. Ok, here goes: say, hostile is above you. In order to engage, you push the mouse away. Your targeting reticle (controlling turrets) slides to approach the hostile, and your ship FOLLOWS.
Inverted mouse style: I used this in WC2 (shuddup, I didn't have a js back then
)
Now I get what you mean: you want that "Up" is triggered by moving the mouse closer to you. But listen: this works only when that targeting reticle is co-axial with your ship (as in FIXED on the centre of your HUD, like in WC2). This is not the case in FL. FL uses free-floating reticle to control the movements of the turrets.
Can you really imagine trying to target an enemy that is above you, while actually pulling the mouse towards you?
I hope this helps