I use a pair of brightly colored primitives, horizontal and vertical stretched boxes or tubes, and just sit them in the scene so they'll provide good contrast when the shaded area of the production window is over them. Make them invisible to the camera, turn off raytracing, shadows, etc... For ease of use, parent them to another primitive and then manipulate that primitive to move them around so you can check various areas of your render to be sure they're in-frame before you render it. For accuracy, parent the planes as BagginsBill suggested. If you're looking for ease of use during changing camera settings, just load up some primitives with bright materials on them.

Yes, this issue bugs me, too. That's why I use the bright primitives and keep a set of them like this in my prop library. :)