Set sorting layer component. Sets the sorting layer and order of any renderer, not just sprite renderers.

SetSortingLayer.cs

using UnityEngine;
using System.Collections;

public class SetSortingLayer : MonoBehaviour 
{
    public string sortingLayerName;
    public int sortingLayerOrder;

    void Start() {
        renderer.sortingLayerName = sortingLayerName;
        renderer.sortingOrder = sortingLayerOrder;
    }
}