gistfile1.rb

# Store somewhere
class SitemapGenerator
  
  def initialize(app)
    @app = app
  end

  def manipulate_resource_list(resources)
    sitemap_builder = ::Middleman::Sitemap::Resource.new(
      @app.sitemap,
      "sitemap.xml", # The URL
      File.join(File.dirname(__FILE__), 'source', 'sitemap.xml.builder') # Path to your template
    )
    resources << sitemap_builder
    resources
  end

end

# In your extension register/setup method
app.after_configuration do
  sitemap.register_resource_list_manipulator(:sitemap_generator,
                                             SitemapGenerator.new(self),
                                             false)
end

Update: Thomas fixed this issue in c9290c6.