singletonizer.rb

module Singletonizer
  def attach_method(name, &block)
    unless respond_to? name
      self.class.class_eval <<-RUBY, __FILE__, __LINE__
        def #{name}(*args)
          if (defined? @attached_methods) && (block = @attached_methods[:#{name}])
            instance_exec(*args, &block)
          else
            ex = NoMethodError.new("undefined method `#{name}' for \#{self.inspect}:\#{self.class}")
            ex.set_backtrace caller(1)
            raise ex
          end
        end
      RUBY
    end
    (@attached_methods ||= {})[name] = block
  end
end

singletonizer_test.rb

require 'singletonizer'

class String
  include Singletonizer
end

a = 'yum'
a.attach_method(:foo) do
  puts self
end

a.foo # works
'blah'.foo # raises error