Unfortunately I do not have experience in billboard advertising. However, I do have experience in massive companies (they suck) as a Software Engineer and I've realized how little they actually know about their own products and the data associated with it. You could probably contact Clear Channel trying to advertise with them and just ask for simple data like expected eye balls/views and population etc. If they have absolutely no idea then you have your answer that there might be a market for this. Your market might not even been the billboard company but rather someone trying to advertise. If I were to advertise I would want some data behind the advertising platform along with some way to track its effectiveness. I assume you are the same @bduerst?
Do you have industry experience with billboard advertising? I might rebuild it if there is an actual demand for it.