This code is designed to hook into unity’s EditorApplication.update delegate.

IRun.cs

namespace CBX.CoreProjectCode.Interfaces
{
    using CBX.CoreProjectCode.Models;

    /// <summary>
    /// Provides a interface for running a <see cref="CallbackModel{T}"/> type.
    /// </summary>
    public interface IRun
    {
        /// <summary>
        /// Runs the <see cref="CallbackModel{T}"/> type.
        /// </summary>
        void Run();                    
    }
}

CallbackModel.cs

namespace CBX.CoreProjectCode.Models
{
    using System;

    using CBX.CoreProjectCode.Interfaces;

    /// <summary>
    /// Provides a modal for callbacks.
    /// </summary>
    /// <typeparam name="T">The generic type used to represent the data type.</typeparam>
    public class CallbackModel<T> : IRun
    {
        /// <summary>
        /// A reference to a callback method.
        /// </summary>
        public Action<T> Callback;

        /// <summary>
        /// A reference to some data that will be passed to the callback method.
        /// </summary>
        public T Data;

        /// <summary>
        /// Implements <see cref="IRun.Run"/> to run the callback. 
        /// </summary>
        public void Run()
        {
            this.Callback(this.Data);
        }
    }
}

EditorCallbackService.cs

namespace CBX.CoreProjectCode.Services
{
    using System;
    using System.Collections.Generic;

    using CBX.CoreProjectCode.Interfaces;
    using CBX.CoreProjectCode.Models;

    /// <summary>
    /// The editor callback service.
    /// </summary>
    public class EditorCallbackService
    {
        /// <summary>
        /// Holds a singleton instance of the <see cref="EditorCallbackService"/> type.
        /// </summary>
        private static EditorCallbackService service;

        /// <summary>
        /// Holds a list of <see cref="CallbackModel{T}"/> types that implement <see cref="IRun"/>.
        /// </summary>
        private readonly Stack<IRun> callbacks;

        /// <summary>
        /// Initializes a new instance of the <see cref="EditorCallbackService"/> class.
        /// </summary>
        public EditorCallbackService()
        {
            this.callbacks = new Stack<IRun>();
        }

        /// <summary>
        /// Returns a singleton instance of the <see cref="EditorCallbackService"/> type.
        /// </summary>
        public static EditorCallbackService Instance
        {
            get
            {
                // if no service yet exists create one
                return service ?? (service = new EditorCallbackService());
            }
        }

        /// <summary>
        /// Runs any callbacks that have been registered.
        /// </summary>
        public void Run()
        {
            while (this.callbacks.Count > 0)
            {
                var cb = this.callbacks.Pop();
                cb.Run();
            }
        }

        /// <summary>
        /// Registers a <see cref="Action{T}"/> callback.
        /// </summary>
        /// <typeparam name="T">The type of data that the <see cref="callback"/> takes as a parameter.</typeparam>
        /// <param name="callback">A reference to a callback.</param>
        public void Register<T>(Action<T> callback)
        {
            this.Register(callback, default(T));
        }

        /// <summary>
        /// Registers a <see cref="Action{T}"/> callback.
        /// </summary>
        /// <typeparam name="T">
        /// The type of data that the <see cref="callback"/> takes as a parameter.
        /// </typeparam>
        /// <param name="callback">
        /// A reference to a callback.
        /// </param>
        /// <param name="data">
        /// The data that will be passed as a parameter when the <see cref="callback"/> is invoked.
        /// </param>
        public void Register<T>(Action<T> callback, T data)
        {
            if (callback == null)
            {
                throw new ArgumentNullException("callback");
            }

            var modal = new CallbackModel<T> { Callback = callback, Data = data };
            this.callbacks.Push(modal);
        }

        /// <summary>
        /// Registers a <see cref="Action"/> callback.
        /// </summary>
        /// <param name="callback">A reference to a callback.</param>
        public void Register(Action callback)
        {
            this.Register<object>(x => callback(), null);
        }
    }
}

Usage scenario …

    [InitializeOnLoad]
    public class EditorInitialization
    {
        /// <summary>
        /// Holds a value indicating whether the RunCallbacks method has been called at least once before.
        /// </summary>
        private static bool ranOnce;

        /// <summary>
        /// Initializes static members of the <see cref="EditorInitialization"/> class.
        /// </summary>
        static EditorInitialization()
        {
            EditorApplication.update += RunCallbacks;
        }

        private static void RunCallbacks()
        {
            if (!ranOnce)
            {
                // setup the settings system
                SetupSettings();

                // load localization strings
                LoadLocalizationData();

                ranOnce = true;
                return;
            }

            // invoke callbacks from editor callback service     
            EditorCallbackService.Instance.Run();
        }
    }