Does this building already have a wallscape on it? If not, you need to check the legality of doing so. Call your city inspections department, and tell them what your plans are and see what they say.
Detroit is a very weak market from an advertising perspective -- maybe the weakest in the nation right now, due to all the negative publicity from the auto industry meltdown. But even in a bad market, you can still find advertisers.
I would first see if you can legally build a wallscape on the building.