On another note; It is only a matter of time before this theory will be tested. As 5Ds get cheaper on the used market, people like us will surely begin to try things such as this. Things we would have never attempted on a $3000 camera.
I plan to do this myself once I can pick up a 5D for $500, and the camera will be worth more to me at that point than whatever photo-gizmos are available at the time. I have a Yashica FX-3 that I got for free with the purchase of a Contax 50/1.7 recently (both for $90!). It will likely be the mirror donor.
On another note; It is only a matter of time before this theory will be tested. As 5Ds get cheaper on the used market, people like us will surely begin to try things such as this. Things we would have never attempted on a $3000 camera.
I plan to do do this myself once I can pick up a 5D for $500, and the camera will be worth more to me at that point than whatever photo-gizmos are available at the time.