Does this work?

Create a config/initializers/pacecar.rb. Inside it:

class ActiveRecord::Base
  include Pacecar
end

No. That gives:

activerecord-2.2.3/lib/active_record/base.rb:2058:in `class_of_active_record_descendant':NoMethodError: undefined method `abstract_class?' for Object:Class

This works, though:

Create a config/initializers/pacecar.rb. Inside it:

module ActiveRecord # :nodoc: all
  class Base

    class << self
      def inherited_with_pacecar(child)
        inherited_without_pacecar(child)
        child.class_eval do
          include Pacecar
        end
      end
      alias_method_chain :inherited, :pacecar
    end

  end
end