MyDecorator.rb

class MyDecorator
  attr_accessor :view_context

  def form(&block)
    view_context.form_for(TheModel.new, &block)
  end
end

ugh.erb

<% my_decorator.view_context = self %>

<h1>The Form</h1>

<%= my_decorator.form do |f| %>
  <fieldset>
    <%= f.text_field :quantity %>
  </fieldset>
<% end %>