I live a couple blocks from Wrigley Field.
Miller Lite has a billboard on one of the rooftops which overlooks Wrigley Field, which you can also view from Sheffield Ave. if you're walking.
They change the sign into a funny slogan that mocks/salutes whichever team they're playing.
Brewers:
http://cdn.faniq.com/images/blog/Picture 27(7).png
Giants:
http://www.thecubdom.com/pictures/2006/09/Picture-297.jpg
Braves:
http://farm2.static.flickr.com/1404/536881547_0c4f5a1fb2.jpg
etc. etc. It changes every series.
If we could get a billboard in a visible part of the city, and make it CLEARLY obvious that it's a Gopher Football billboard. I think it would be awesome to get something similar to those old school homecoming pins where there's a pun on the team we're playing that week.
People would drive past it on Monday mornings on their way to work, chuckle at the slogan for the week, know who's on the schedule, and maybe buy tickets.
That's how I think a billboard could actually work.