I can't reproduce this problem. The other cameras that are used as projection cameras do change their aspect ratios, but the aspect ratio isn't used by the Image Map Shader. The aspect ratio is controlled by the size parameters in the shader's "Projection, Location" tab.
Can you show us some screenshots or renders which show the differences you're seeing when you change the pixel aspect ratio?
If you mean that the projected image appears to be stretched differently after you have changed the pixel aspect ratio of the renderer, then it should stretch in the same way as everything else stretches because it's part of the world which is being viewed by your renderer. The stretched image should line up with the stretched clouds as it did before.
Matt