Not sure about doc example. For the demo service, I put a lambda into 
    drds_collection = {
      total_count: drds.count, 
      items: drds,
      location_options: lambda {|h| {'list' => ['option1', 'option2a']} }
    }
in drds.rb and the factory maps the method call to the lambda.
I limited it to _options. Technically that isn't hard to unlimit, but I'm not sure if that is wanted.