a.rb

class Base
  class Config
    def id(value)
      @id = value
    end

    def to_s
      @id
    end
  end

  def self.config(&blk)
    @@config ||= Config.new
    @@config.instance_eval(&blk)
  end

  def config
    @@config
  end
end

class This < Base
  config do
    id "hello1"
  end
end

class That < Base
  config do
    id "hello2"
  end
end

puts This.new.config.to_s # => hello2
puts That.new.config.to_s # => hello2