presenter helper method with yield, basic idea taken from Ryan Bates https://github.com/railscasts/287-presenters-from-scratch/blob/master/profile-after/app/helpers/application_helper.rb#L2

gistfile1.rb

# some ./app/helpers/some_helper.rb, in my case actually in an engine gem
module SomeGemHelper
  def my_gem_decorate(model)
     # Here I'm hard-coding to always decorate with MyDecorator,
     # but it could also be passed in as a method arg, or guessed
     # from the model.class name, or from a differnet model attribute
     # like model.presenter_class, or taken from configuration, or
     # some combination -- whatever meets the needs of your design. 
     decorated = MyDecorator.new(model, self)
     yield if block_given?
     decorated
  end
end

gistfile2.rhtml

# Now in any old view template, check this out:

<% my_gem_decorate(@model_obj) do |model_obj| %>
    <p><%= model_obj.decorator_method %></p>
    
    <%# Pass it to a partial, the partial gets the decorated
      # object, great. %>
    <%= render "some_partial", :object => model_obj %>
    
<% end %>