Hmm... I actually have my doubts about the power of signs asking for songs. It's really rare that U2 will play something that they haven't rehearsed, unless it's a really simple song or they have played it not long ago. So I think that fans with signs can encourage them and give them ideas, but it's unlikely they will play the requested song all of a sudden, unless they had planned to do it anyways.
Asking them before the show has a greater chance IMO, because if they want to play it they can always do a quick rehearsal during soundcheck. Apparently this is what happened for Fast Cars nad The First Time.