a.html.erb

<% A.foo(@user) do |f| %>
  <%= f.label :name %>
  <%= f.text_field :name %>
<% end %>

foo.rb

module A
  def self.foo(obj, options = {}, &block)
    # もっと簡単な方法は無いものか...
    options[:url] ||= Rails.application.routes.url_helpers.polymorphic_path(obj, format: :json)
    options[:builder] ||= CustomBuilder

    # これも良いのか微妙
    ApplicationController.helpers.form_for(obj, options, &block)
  end
end