I listened to Seth Godin's presentation on his book "Tribes" recently, and one of the many useful things I got out of it is that we are really in the post-advertising age. The idea that throwing a branded message at a selected market over and over until it gives in and purchases, is dying, because we're all overloaded with channels and alternatives and avoidance technology like Tivo. We just turn off.
Top down: The city (who does this thing belong to, anyway?) or some other public institution designates someone to keep this thing full of wonderful content. That person is an editor, and they gatekeep what goes on the media tower - it's exclusive, so everyone wants to make something that 'deserves' the exposure. I daresay the higher the artistic standards, the more successful it will be.
Bottom up: Everybody gets to put something up, for a price. Sometimes PDAs go up, sometimes inspirational messages, sometimes marriage proposals. I don't think this could be profitable, because there's a lot to look at, and the challenge in designing billboards and signage is keeping the message simple enough to communicate to the speed of travel. Even for pedestrians.