board_controller.rb

class BoardController < ActionController::Base
  before_filter :build_nav_bar_links
  
  def index
    ...
  end
  
  private
    def build_nav_bar_links
      @nav_bar_links = nav_bar_links
    end
  
    def nav_bar_links
      [["Something", something_url],
        ["Else", else_url]]
    end
end

init.rb

class BoardController
  include RandomPlugin::NavBarExtensions
end

nav_bar_extensions.rb

module RandomPlugin
  module NavBarExtensions
    def nav_bar_links_with_random_plugin
      links = nav_bar_links_without_random_plugin
      links << ["Extra", extra_url]
    end

    def self.included(receiver)
      receiver.alias_method_chain :nav_bar_links, :random_plugin
    end
  end
end