There isn't a maintenance mode in DNN. What I would do is the following.
Setup a new website in IIS, beta.mywebsite.com. Have that website point to your Existing DNN folder.
Point your current website to a new folder with the App_Offline.htm file/message in place. Then have your admins go to the beta.mywebsite.com URL instead of going to the www version of the URL.
That would probably be the most straightforward way to do this for DNN without writing a custom maintenance mode module for DNN.