In progress - wanting to generalise this solution for multiple sub-configuration types...

AppServerConfiguration.rb

class AppServerConfiguration
  attr_accessor :port, :admin_password
end

class FooServerConfiguration
  attr_accessor :moo;
end

class Configuration
    attr_accessor :tail_logs, :max_connections, :admin_password 
                   
    def initialize
      @foo = AppServerConfiguration.new
      @moo = FooServerConfiguration.new      
    end
    
    def app_server(&block)   
      if block_given?
        block.yield(@foo)
      end
      @foo
    end  

    def foo_server(&block)    #Yuk - DRY!
      if block_given?
        block.yield(@moo)
      end
      @foo
    end  
end

def configure(&block)
  foo = Configuration.new
  block.yield(foo)
  return foo
end
 
configuration = configure do |config|  
  config.tail_logs = true  
  config.max_connections = 55  
  config.admin_password = 'secret'  
  config.app_server do |app_server_config|  
    app_server_config.port = 8808  
    app_server_config.admin_password = config.admin_password  
  end        
  config.foo_server do |foo_server_config|
    foo_server_config.moo = 888;
  end
  config.app_server do |app_server_config|  
     app_server_config.port = 9000
   end
end  
  
puts configuration.class # => Configuration  
puts configuration.tail_logs # => true  
puts configuration.app_server.admin_password # => 'secret
puts configuration.app_server.port # => 9000