It seems as if the Cubs are getting more serious about putting up at least one of those signs, because Wednesday, using two cranes and ropes held by several workers, they put up a sample right-field sign that had the words "Wrigley Field" on it, in see-through fashion. (Obviously, this would be replaced with a sponsor's name if actually constructed.)
Several members of Cubs management and rooftop owners gathered on the Skybox on Sheffield rooftop to observe as the sign was moved to several different positions along Sheffield, varying from very close to the back of the party patio, to all the way along the east side of the street -- it seems very unlikely that they would or could place it that far back.
From what I could tell, it didn't seem as if the views from any of the seats actually on that rooftop -- likely the only one that would be even partially blocked by such a sign -- would have any trouble still seeing the field. It's possible that some views from levels below the rooftop might have some blockage, but then, people who are inside are even less likely to be paying attention to the game than those on the actual rooftop.
I have reached out to the Cubs for comment on what happened today, but have not yet heard back from them. If and when I do, I'll add that to this post.
UPDATE: The Cubs got back to me with the following statement:
We did a mock up of what the eventual sign in right field will look like which will be activated by Budweiser. It also provided an opportunity for our rooftop partners to see what the sign could look like. Given the size of the sign which has been reduced from 1,000 to 650 square feet, the location of the sign and the plan to move the walls back on Sheffield, every rooftop partner on Sheffield will be able to have views in the ballpark.
Here's some video I took of them raising the sign (WARNING! You might want to turn your speakers down before you play this as the noise from the crane is pretty loud, and I was standing right next to it).