Anything hanging in a breezeway like that is a disaster waiting to happen. Even with security cameras, those places are hangouts for kids on skateboards, bikes whatever. It will get ruined if it's not out of reach. Besides, like I said, the wind kicks up in those places. Unless they are willing to give up the second lane completely and dedicate it to a permanent sign which can be changed out monthly as requested, you're fighting a losing battle.
Either off to the side where there is no traffic, a dedicated area and behind a closure or up on the fascia. Other than that, you're just tempting fate and the kids to do their nasties on it.
If you put a height restriction on that lane, you'll be covering your butt, but I just saw a bank near where I live have to completely remove and put a whole roof on a set up exactly like yours this past summer. They had pictures/video of the truck taking out the roof, by about 2" or 3". They had it posted, but guess what..... not in Spanish. Insurance had to cover it.