A concept I just had....
A map, right - that uses a wad.
A VB application
Every time you start the app, it connects to a server somewhere and downloads the latest version of the wad. The wad would only need to have one texture in it, the advert. The wad is then copied into the "Valve" directory.
Thats it. Since the wad is loaded when you start the map, the textures in the map would be updated.
You could do the same with sentences.txt, titles.txt or anything else.
Poor man's Steam!