gistfile1.rb

# This file provided in engine

module MyEngine
   module MainAppControllerMethods
      def do_it      
        return params[:do_it]
      end
   end
end

# Local 'main app' should be instructed to simply add this manually to their
# own ApplicationController

require 'my_engine/main_app_controller_methods'
class ApplicationController < ActionController::Base
   #....
   
   include MyEngine::MainAppControllerMethods

   # ....
end

# Now every controller in the host app has access to a #do_it method.