SplitPolygonColliders.cs

using UnityEngine;
using UnityEditor;
using System.Collections.Generic;
using System.Linq;
using System;

[Tiled2Unity.CustomTiledImporter]
public class SplitPolygonColliders : Tiled2Unity.ICustomTiledImporter {

    public void HandleCustomProperties(GameObject gameObject, IDictionary<string, string> customProperties)
    {
        // do nothing
    }

    public void CustomizePrefab(GameObject prefab)
    {
        var polygon2Ds = prefab.GetComponentsInChildren<PolygonCollider2D>();
        if (polygon2Ds == null)
            return;

        int wallMask = LayerMask.NameToLayer("Walls");
        var wallPolygons = from polygon in polygon2Ds where polygon.gameObject.layer == wallMask select polygon;


        foreach(var poly in wallPolygons)
        {
            for (int p = 0; p < poly.pathCount; p++)
            {
                // make a new gameobject for the collider
                GameObject newCollider = new GameObject("Collider_" + p);
                newCollider.transform.parent = poly.transform;

                // Make the PolygonCollider2D
                PolygonCollider2D newPolygonCollider2D = newCollider.AddComponent<PolygonCollider2D>() as PolygonCollider2D;
                newPolygonCollider2D.SetPath(0, poly.GetPath(p));
                newPolygonCollider2D.transform.parent = poly.gameObject.transform;
            }
        }
    }
}