private-block.rb

module PrivateBlock
  def private(*args, &block)
    return super unless block_given?

    begin
      methods = []
      mod = Module.new do
        define_method :method_added do |name|
          super(name).tap do
            methods << name
          end
        end
      end
      self.singleton_class.prepend mod
      block.call
    ensure
      private(*methods)
      mod.remove_method :method_added
    end
  end
end

class Module
  prepend PrivateBlock
end

class A
  private do
    def foo
      p 'foo'
    end
  end

  def bar
    p 'bar'
  end
end

A.new.foo rescue p $!
A.new.__send__(:foo)

class B
  def self.method_added(name)
    puts "#{name} is defined!"
  end

  private do
    def foo
      p 'foo'
    end
  end

  def bar
    p 'bar'
  end
end

B.new.foo rescue p $!
B.new.bar