As I was sitting here debating on taking an old Tamron lens apart, I had an idea about camera/lens design. The idea is to put the aperture diaphragm inside the camera body near the lens mount, instead of the rear of the lens.The physical aperture needs to be at a place in the optical train substantially away from where the rays from a point on the object converge.
For example, to take it to an extreme, suppose the aperture were put just in front of the sensor. Then it would not affect the illuminance across the image (the exposure), but would merely adjustably mask the image.
This would apply to some degree at any point from the flange (i.e. inside the body). Especially for telephoto lenses, the aperture diaphragm needs to be somewhere near the front of the lens. (My understanding is that the exact position required depends on the lens design.)
Another point to consider - to amplify Doug's point, is to think of how large the aperture needs to be. On an 85mm f/1.2 lens, the aperture is 70.8mm in diameter when open. - Anywhere close to the focal plane it would no effect.
On a 17-40mm f/4 lens, and 17mm the aperture is 4.25mm in diameter when completely open. Anywhere near the focal plane, that would just act as a mask, as Doug mentions.
Now, for a 400mm f/2.8 lens the aperture is 142.86mm in diameter when completely open!