snippet.rb

class Object
  def self.method_added(name)
    return if name == :initialize
    const_set(:InstanceMethods, Module.new) unless defined?(self::InstanceMethods)
    self::InstanceMethods.send(:define_method, name, &instance_method(name).bind(self.allocate))
    remove_method(name)
    include self::InstanceMethods
  end
end

class A
  def initialize; @a = "X" end
end
class B < A
  def foo; @a end
end

p B.new.foo # => nil