I've created a slightly more workable solution, merging the movement and camera scripts together. Now, the "ball centre" object that the camera is attached to controls turning, while forward and backwards controls apply a force on the ball in the direction that the camera is facing.

using UnityEngine;
using System.Collections;

public class CameraScript : MonoBehaviour 
{
    public Transform ballObject, ballCamera;
    public float speed, turnSpeed;
    Vector3 forceDirection;

    void FixedUpdate() 
    {
        transform.position = ballObject.position;
        if(Input.GetAxis("Vertical") != 0)
        {
            RollBall();
        }
        //calculate ball turning
        transform.Rotate((Vector3.up * Input.GetAxis("Horizontal") * turnSpeed), Space.World);
    }

    void RollBall()
    {
        //set force down camera's look direction
        forceDirection = ballCamera.transform.forward;

        //remove y force direction for angled camera
        forceDirection = new Vector3(forceDirection.x, 0, forceDirection.z);

        //add force to ball
        ballObject.rigidbody.AddForce(forceDirection.normalized * speed * (Input.GetAxis("Vertical")));
    }
}

The result is that the movement can be considered in a way that is independent of the rotation of the ball - so it's more of a work-around than a solution, but there didn't seem to be a graceful way to clone a rotation value across via a Vector3.

If what you want is just to keep the camera facing always the direction of movement, you must calculate its direction based on the last position. To avoid inaccuracies produced by very small displacements, you can set a minimum distance to refresh the view direction (adjust the distance variable in the Inspector if needed):

using UnityEngine;
using System.Collections;

public class CameraScript : MonoBehaviour 
{
    public Transform ballObject;    
    public float distance = 0.1f; // min distance to refresh view direction

    Vector3 lastPos;
    Vector3 viewDir;

    void Start(){
        // ensure forward direction at start
       lastPos = ballObject.transform.position - transform.forward * distance; 
    }   

    void Update() 
    {
       Vector3 ballPos = ballObject.transform.position;
       Vector3 newDir = ballPos - lastPos;  // direction from last position
       newDir.y = 0; // keep the camera on horizontal plane
       if (newDir.magnitude>distance){  // only recalculate after min distance
         viewDir = newDir;
         lastPos = ballPos;
       }
        transform.position = ballPos;
       transform.forward = Vector3.Slerp(transform.forward, viewDir.normalized, Time.deltaTime);
    }
}

Try this:

using UnityEngine; using System.Collections;

public class CameraScript : MonoBehaviour { public Transform ballObject;

void Update() 
{
    transform.position = ballObject.position;
    transform.rotation.eulerAngles = new Vector3(0, ballObject.transform.rotation.y, 0);
}

}

You don't need to add the .transform after ballObject because it is a transform.