Draw texture on object, with a rotating camera

Hi everyone,

I'm making a first person spatial shooter in cockpit view. I need a lock on system to track ennemies on screen. My first idea was to get the position of the object I need to lock on, convert it to screen position, and draw on that, like that :

Code (CSharp):

1.  
2. void OnGUI() {
3.             Vector3 position = Camera.main.WorldToScreenPoint (target.transform.position);
4.             GUI.DrawTexture (new Rect (position.x,position.y, 0, 0), lockImage);
5. }

Unfortunatly, as the camera is always rotating (chasing ennemies in space, and stuff), it's not enough. I tried to correct the position with rotation matrix, but I don't seem to get it right. Is it the way to go ? Or is there another, easier, solution ?

Thanks,

Etienne