RenderTextureTest.cs

using UnityEngine;
using System.Collections;

public class RenderTextureTest : MonoBehaviour
{
    public Transform quad;
    public Camera ingameCamera;

    public bool checkObstruction;

    void Start ()
    {
        UICamera.raycastConversion = RenderTextureRaycastConversion;
    }

    public Vector3 RenderTextureRaycastConversion(Vector3 screenPoint)
    {
        Ray ray = ingameCamera.ScreenPointToRay(screenPoint);

        Debug.DrawRay(ray.origin, ray.direction * 100, Color.green);

        // raycast to the quad plane
        Plane plane = new Plane(quad.transform.forward, quad.transform.position);

        float enter;
        if (!plane.Raycast(ray, out enter))
            return new Vector3(float.NaN, float.NaN, float.NaN);

        Vector3 projectionPoint = ray.origin + ray.direction * enter;

        if (checkObstruction)
        {
            if (Physics.Raycast(ray, enter - 0.1f))
                return new Vector3(float.NaN, float.NaN, float.NaN);
        }

        Debug.DrawLine(Vector3.zero, projectionPoint, Color.red);

        // transform to quad space
        Vector3 quadSpace = quad.transform.InverseTransformPoint(projectionPoint);

        Vector3 finalPoint = quadSpace + new Vector3(0.5f, 0.5f, 0);
        return finalPoint;
    }
}