Usage :: Attach to UIPanel(NGUI) Object.

PointingBillboard.cs

using UnityEngine;
using System.Collections;

[RequireComponent (typeof(UIPanel)) ]
public class PointingBillboard : MonoBehaviour {

  public GameObject TargetObj;
	public Vector3 Offset;
	private Vector3 _latestOffset;
	private Vector3	_targetPos; //GameObject's position.
	private Vector3 _targetLatestPos;
	private Camera	_2DCam;
	private Camera	_3DCam;

	/* FORCE Stay current position  */
	private bool _forceStay;
	public bool forceStay{	set{this._forceStay = value;} }
	// Use this for initialization
	void Start () {
		_2DCam = GetMyCamera(transform);
		_3DCam = Camera.main;
	}
	
	/*再起的に親オブジェクトにカメラが無いか確認。あればカメラを返す。*/	
	Camera GetMyCamera(Transform tr){
		GameObject pr = tr.parent.gameObject;
		Camera cam = pr.camera;
		if(cam != null){
			Debug.Log("cam:" + pr.gameObject.name);
			return cam;
		}else if( pr != null ){ 
			return GetMyCamera(pr.transform);
		}else{
			Debug.Break();
			Debug.LogWarning("NGUI Camera doe's not found.");
			return null;
		}
	}

	// Update is called once per frame
	void Update () {
		SnapToTargetGO();
	}
	void SnapToTargetGO(){
		Vector3 targetObjPosAt2D = _3DCam.WorldToScreenPoint(TargetObj.transform.position);
//		Vector3 targetObjPosAt2D = _3DCam.WorldToScreenPoint(TargetObj.position);
		/* if nothing between latest position and current position, do nothing. */
		if( targetObjPosAt2D == _targetLatestPos &&
			Offset == _latestOffset) return;
		_targetLatestPos = targetObjPosAt2D;
		_latestOffset = Offset;
		Vector3 posAt2DWorld = _2DCam.ScreenToWorldPoint(targetObjPosAt2D);
		transform.position = new Vector3(posAt2DWorld.x + Offset.x,posAt2DWorld.y+Offset.y, transform.position.z);
	}

}

PointingBillboardInspector.cs

using UnityEngine;
using UnityEditor;
using System.Collections;
[CanEditMultipleObjects]
[CustomEditor (typeof(PointingBillboard))]
public class PointingBillboardInspector : Editor {
  public override void  OnInspectorGUI(){
		var pb        = target as PointingBillboard;
		pb.Offset     = EditorGUILayout.Vector3Field("Offset", pb.Offset );
		pb.TargetObj  = EditorGUILayout.ObjectField("target",pb.TargetObj,typeof(GameObject)) as GameObject;
	}
}