This would live in the lib directory of a gem. That gem would also provide some JS and some sort of controller logic about how to handle the data / form fields.

form_ext.rb

module FormExt
  module FormHelper
    def self.included(arg)
      ActionView::Helpers::FormBuilder.send(:include, FormExt::FormBuilder)
    end
    def multi_valued_text_field_tag(object_name, method, options = {})
      output = ''
      if options and options[:value] and options[:value].is_a?(Array)
         options.merge!(:"data-multivalued" => true)
         options[:value].each_with_index do |value, index|
           options.merge!(:name => "#{object_name}[#{method}][]")
           options.merge!(:id => "#{object_name}_#{method}_#{index}")
           output.concat text_field(object_name, method, options.merge(:value => value))
           output.concat "<input type='button' value='Delete' name='#{object_name}_#{method}_#{index}' data-delete-multival='true' />" unless index == 0
           output.concat options[:delimiter] if options[:delimiter]
         end
      else
        output.concat text_field(object_name, method, options)
      end
      output.concat "<br/><input type='submit' value='Add #{method}' name='add_#{object_name}_#{method}' data-add-multival='true' />" unless options[:from_form_builder]
      output.html_safe
    end
  end
  
  
  module FormBuilder
    def multi_valued_text_field(method, options = {})
      output = ''
      object = object_from_options(options)
      merge_data_attribute(object, method, options)
      if options[:"data-multivalued"]
        values = object.send(method)
        values = [values] unless values.is_a?(Array)
        values.each_with_index do |value, index|
          options.merge!(:name => "#{object_name}[#{method}][]")
          options.merge!(:id => "#{object_name}_#{method}_#{index}")
          output.concat @template.multi_valued_text_field_tag(@object_name, method, objectify_options(options).merge(:value => value, :from_form_builder => true))
          output.concat "<input type='submit' value='Delete' name='delete_#{object_name}_#{method}_#{index}' data-delete-multival='true' />" unless index == 0
          output.concat options[:delimiter] if options[:delimiter]
        end
      else
        output.concat @template.multi_valued_text_field_tag(@object_name, method, objectify_options(options).merge(:from_form_builder => true))
      end
      output.concat "<br/><input type='submit' value='Add #{method}' name='add_#{object_name}_#{method}' data-add-multival='true' />"
      output.html_safe
    end  
    
    private
    def object_from_options(options)
      objectified_options = objectify_options(options)
      return objectified_options[:object] unless objectified_options[:object].blank?
    end

    def merge_data_attribute(object, method, options = {})
      if object# and object.send(method).is_a?(Array)
        options.merge!(:"data-multivalued" => true)
      end
    end
    
  end
end

ActionView::Helpers::FormHelper.send(:include, FormExt::FormHelper)