add comments to ALL models (ie automatically add has_many)

I have the acts_as_commentable and a bunch of others added to pretty
much every model, is there a way to automaically add these to EVERY
model ?

I tried the following but it fails with an error...

ActiveRecord::Base.class_eval do
  has_many :notes, :as => :model_with_notes, :dependent => true

fails with....
base.rb:1369:in `class_of_active_record_descendant':NoMethodError:
undefined method `abstract_class?' for Object:Class

Without knowing the details, I would suggest using inheritance.
Instead of trying to change ActiveRecord::Base, just derive a new
class from it to hold all the common stuff, then derive your models
from that class.