Use OnMouseEnter/OnMouse Exit with center of screen

The thing is, I wanna do something looking like this: Your cursor is locked in the center of the screen and looks like a crosshair. The thing is: I know how to freeze the cursor but I haven't found any way to set it position, and when showcursor is set to false, OnMouseOver and OnMouseExit doesn't work.

Here is my code:

 public bool display = false;
 public string text = "";
 public void OnGUI()
 {
     GUI.backgroundColor = Color.white;
     GUI.color = Color.cyan;
     GUI.Label (new Rect(Screen.width/2-20, Screen.height/2-50, 100, 50), text);
 }
 public void OnMouseOver ()
 {
     Debug.Log (Vector3.Distance (Camera.main.transform.position, transform.position));
     if (Vector3.Distance (Camera.main.transform.position, transform.position) < 3) 
     {
         text = "Interact";
         renderer.material.shader = Shader.Find("Outlined/Silhouetted Bumped Diffuse");
     } 
     else 
     {
         text = "";
         renderer.material.shader = Shader.Find("Diffuse");
     }
 }
 
 public void OnMouseExit ()
 {
     Debug.Log ("Exit");
     renderer.material.shader = Shader.Find ("Diffuse");
     text = "";
 }

Is there anyway to make OnMouseOver and OnMouseExit work with the center of the screen?