PointerCursor.jslib goes in Assets/Plugins/WebGL, PointerCursor.cs goes in your scripts folder, call PointerCursor.SetPointer(true) to set the cursor as a pointer

PointerCursor.cs

using UnityEngine;
using System.Collections;
using System.Runtime.InteropServices;

public class PointerCursor : MonoBehaviour {

    [DllImport("__Internal")]
    public static extern void SetPointer(bool arg);
}

PointerCursor.jslib

var PointerCursor = {
    SetPointer: function(arg)
    {
		document.getElementById("canvas").style.cursor = arg ? "pointer" : "default";
		console.log("Set Cursor");
    }
};

mergeInto(LibraryManager.library, PointerCursor);