And when you say 100-400 mm lens, I guess you are referring to the focus relative to the analogue technology? I've never read up on digital SLR technology, but I understand the focal length needs to be reduced by about 1/3rd for the same results, because the CCD is smaller?
It depends on the DSLR. The Canon EOS 300D for example will increase the focal length of the image 1.3x on top of what the lense gives you. The more expensive 'full frame' Canon EOS 5D will not though and the focal length the lense is set to is the focal length you get.
I've never tried this (although I was told by an instructor that it can be done and it works quite well) but on the Super Cub you can open the door in flight (it's a split door with on half folding down and the top half latching onto the underside of the wing) which gives you a very large space in which to swing a camera. The sliding window on the left side is also useful although avoiding getting the struts in the picture is quite difficult.