This version is a bit rough and crap lol... the one I'm working on basically uses the concept but instead of reading the maintenance config, the add-on comes with a config it reads and applies from there, but no to install add ons you will still have to restart or change map (depending on the add-on) but the main reason is so that members/users don't join constantly...
But it will change the server name, to example, my server name would change to " Thirdage Gaming | Maintenance " and set a password to it (of your choice) which they (hopefully) will not find out.