well, basically what it would have to do would be intercept the disconnect signal and send it a download command instead. This would imply that the connection would have to stay open which might cause issues. I'm sure it can be done, I'm just saying. If nothing else, it could easily print out to the URL's of the files needed for the user and where they need to go. This is probably the first step. After that works, make it so it functions as a download. For security reasons, you should really ask if the user really wants to download file x and the creators of the mod are not liable for any damages that may result from downloading malicious files.
EDIT: this also means that all map packs capable of being autodownloaded must have the same folder structure when downloaded. It's can't vary from person to person and no .txt files included with the maps. It would cause excess clutter for the player.