cutpoints.rb

def create
  @obj = SomeClass.create(params[:shmoo])
  execute_callbacks :on => :shmoo_creation, :with => @obj
  redirect_to somewhere
end


def execute_callbacks(options)
  @callbacks ||= {}
  event_name = options.delete(:on)
  if @callbacks[event_name]
    @callbacks[event_name].each{|c| c.call(options[:with]) }
  end
end

def add_callback(on_event, &with_block)
  @callbacks ||= {}
  @callbacks[on_event] ||= []
  @callbacks << with_block.to_proc
end


class SomeController <<
  before_filter do | c |
      get_all_plugins.each do | plugin |
        plugin.new(c)
      end
  end
end

# А вот это твой плагин
class ShmooMailer
  def initialize(with_controller)
    with_controller.add_callback(:shmoo_creation) do | shmoo |
      SuperMailer.deliver_greeting_to_shmoo_owner(shmoo)
    end
  end
end