If a dedicated server you can customize the settings screens a message to entering the server, welcome for example or any other. As admin, you also have the freedom to other actions.
I do not understand. You can customize the message in two ways: Dedicated Server Settings or file "multiplayer.ini" for the same.
Include the message in field "MOTD" (Message of the day).
I believe it doesn't appear, no. I don't think it is possible to auto announce an advert (which personally is better - I'd hate to join a server and get an advert, especially if it broadcast to everyone every time someone joined)
My rationale behind the post is that we are currently short on a few racers for a league yet the public server is always full. I would like the ability to send an automated recruitment message every 15 mins or so
That would be bad, especially if I was racing, to see that pop up so often... just saying. But I get what you mean. You can post your community details here, which may help (if you haven't already done so) and put your website name in the server name; serious sim racers will then go to the site by themselves if they want more like leagues. Not a perfect solution, but better than nothing I suppose