I like the look that is possible with zone plate photography. I've google around a bit and it seems that pinhole and zone plate photography on a DSLR, especially a crop body DSLR, results in medium telephoto type shots. It also seems that the focal length is dependent upon the distance from the plate (or pinhole) to the sensor.
You could take a body cap and inset it some into the camera body, but it will eventually strike the mirror. Going one step further, I could use mirror lockup and and get the zone plate even closer to the sensor. If I do that, what sorts of problems am I likely to see. And this raises an interesting question, does the Canon 40D even have a mechanical shutter and would this inset device interfere with that?