SortingLayerOffsetter.cs

// Sorting Layer Offset, for Unity sprites. By Jon Manning (@desplesda) at Secret Lab! http://secretlab.com.au

// This code is released under the CC-0 license. https://creativecommons.org/publicdomain/zero/1.0/
// You can do whatever you like with it, but neither I nor Secret Lab offer any support for it.

using UnityEngine;
using System.Collections;

// Shift all sorting layers based on Z position - the farther they are in Z,
// the lower their sorting order will be!

public class SortingLayerOffsetter : MonoBehaviour {

    // Increase this if you need sorting layers to be offset more
    static int multiplier = 10;

    // Do this first so that the player never suspects a thing
    void Awake () {

        // Figure out how far into the back we're moving each sprite
        int offset = (int)(transform.position.z * multiplier);

        // Get every single sprite renderer
        var allSpriteRenderers = GetComponentsInChildren<SpriteRenderer>(includeInactive:true);

        // Shift it back based on how far it is in Z
        foreach (var renderer in allSpriteRenderers) {
            renderer.sortingOrder -= offset;
        }

        // NICE.
    }
}