init.rb

# vendor/plugins/plugin_a/init.rb
Dispatcher.to_prepare do
  require_dependency 'application' # for ApplicationController
  ApplicationController.send(:include, PluginAPatch)
end

plugin_a_helper.rb

# vendor/plugins/plugin_a/app/helpers/plugin_a_helper.rb
module PluginAHelper
  def a_new_method
    "You've been patched!"
  end
end

plugin_a_patch.rb

# vendor/plugins/plugin_a/lib/plugin_a_patch.rb
module PluginAPatch
  def self.included(base)
    base.class_eval do
      helper :plugin_a
    end
  end
end