Just a little update for auto-dl dyn ad.
The only idea I had, is to create a fake map containing the texture.
So you must put
metro-DA before metro-b1-2 in your rotation conf. By this way, client will dl the fake map (just a room with no spawn).
Replace /textures/dyn_ad/dyn_ad_80x48.jpg in the pk3 to have your own ad (keep the same name).
Also, like biotxc said, give the pk3 an unique name (for ex: map-metro-DA-MyServer.pk3) to avoid conflict with other server file.
The only problem with this, is: if a client join the real map, he won't have the updated pk3.