You're right. It's not working at all, whether ray trace objects is on or off. I will fix this.
There is a workaround. How perfect does your orthographic projection need to be? You can recreate a nearly orthographic view by placing the camera very high and zooming in. Try this: Choose Perspective mode. Set the focal length to 36000, and the film aperture to 36, 36. Now because the focal length is 1000 times the aperture, put the camera at a height of 1000 times the orthographic area you want to render. For example, if I want to render an area 500 metres on each side, I set the camera Y to 500000. With these settings the field of view is narrow enough that perspective distortion is almost non-existent, and you should get an almost perfect orthographic view.
Another way you can do this is to enter the orthographic area into the film aperture and then enter the height of the camera into the focal length, but you have to make sure the camera is high enough that this produces a very narrow field of view. You can ignore the fact that it's labeled in mm because the only thing that really matters here is the ratio of focal length to aperture (although the mm label will become important when we add depth of field rendering in future).
The only problem with this method arises if you need to place the camera closer to the ground for some reason, perhaps to change how the atmosphere is rendered, but hopefully you don't need to do that.
Matt