public class CustomHttpControllerSelector : DefaultHttpControllerSelector {
  private readonly HttpConfiguration _configuration;

  public CustomHttpControllerSelector(HttpConfiguration configuration) : base(configuration) {
    _configuration = configuration;
  }

  public override HttpControllerDescriptor SelectController(HttpRequestMessage request) {
    HttpControllerDescriptor controller;
    try {
      controller = base.SelectController(request);
    }
    catch (Exception ex) {
      String controllerName = base.GetControllerName(request);
      Assembly assembly = Assembly.LoadFile(String.Format("{0}pak\{1}.dll", HostingEnvironment.ApplicationPhysicalPath, controllerName));
      Type controllerType = assembly.GetTypes()
        .Where(i => typeof(IHttpController).IsAssignableFrom(i)).ToList()
        .FirstOrDefault(i => i.Name.ToLower() == controllerName.ToLower() + "controller");
      controller = new HttpControllerDescriptor(_configuration, controllerName, controllerType);
    }
    return controller;
  }
}

config.Services.Replace(typeof(IHttpControllerSelector), new CustomHttpControllerSelector(config));