It wouldn't require remapping. I think the easiest way would be to create a low polygon model of a banner with skins for each of the different advertisements. The model could be injected into the map by creating an .ent file for it, or having it injected when the map loads by the game dll. Using QuArK (since it can load .bsps directly) you can tweak the coordinates of each banner in different maps, and have them saved in individual ents, or store the map/coord combinations directly in the game binary so people (server admins) don't just hack the ads out.