nokogiri template handler.rb

require 'action_view'

module ActionView
  module Template::Handlers
    class NokogiriBuilder
      class_attribute :default_format
      self.default_format = Mime::XML

      def call(template)
        "xml = ::Nokogiri::XML::Builder.new { |xml|" +
          template.source +
          "}.to_xml;"
      end
      
      protected
      def require_engine
        @required ||= begin
          require "nokogiri"
          true
        end
      end
    end
  end
end

ActionView::Template.register_template_handler :nokogiri, ActionView::Template::Handlers::NokogiriBuilder.new

#Placing files like this in the lib directory is sufficient. If you have many builders(handlers), then adding a lib/builders/ or lib/handlers/ directory may be better. Note that you'll need to add those paths to your application config with something like:
#add this to application.rb This will add all subdirectories in lib to your application auto load path. You could just add the specific paths you want to require.
config.autoload_paths += Dir[ "#{config.root}/lib/**/" ]
#The template file extension is your choice. In this gist, NokogiriBuilder.new is registered to handle files with the extension :nokogiri