If your software guys are up for some heavy image processing, they can use the resources linked by davidthefat in
this thread. They describe the Hough algorithm, which constructs a polygon from a series of points in an image. If you use an algorithm that can construct a polygon instead of looking for an ellipse or rectangle, you don't have to worry as much about perspective distortion affecting detection, because even if you're at an angle that makes the vision target look like a parallelogram instead of a rectangle, the algorithm will be able to tell that it's there.