Sure, if / when the production comes to Broadway, the artwork will most likely be different. But the (as you call it) 'placeholder' artwork for Shrek was used all through their Seattle run.
If you look at the 5th Ave website, all their 2009-2010 shows have similar logos because their ad gimmick for next season has been that it's a season of "technicolor dreams." Each show has a color associated with it (CMIYC is green for money) that then is the logo background with a simple font - they definitely won't be using those in the actual runs of the shows, though. They never have their actual poster art this early.
If you look at the 5th Ave website, all their 2009-2010 shows have similar logos because their ad gimmick for next season has been that it's a season of "technicolor dreams." Each show has a color associated with it (CMIYC is green for money) that then is the logo background with a simple font - they definitely won't be using those in the actual runs of the shows, though. They never have their actual poster art this early.
The 5th traditionally had a 25 or under rush, but I haven't seen it advertised recently.
Your best bet is to become a "Friend" of the 5th Ave through their FaceBook page, and they'll sometimes post up specials that they have on there, but it's usually "the day of" specials that don't have all the time, and usually for low traffic days (Tuesday and Wednesday nights)
There also used to be a "day of show" tickets window in the Pacific Place in downtown Seattle, but they closed this last month, and I don't know if there is anywhere else in Seattle where they do that.