Did you ever find the problem. If you put the camera on the scope with no film, open the back and set the shutter speed to 4 seconds. Will the shutter fire and the mirror come up so that you can see through the camera into the scope?

Some electronic cameras will not fire the shutter without a proper electronic cable release when the exposure time increases past a certain point.

I use an old Petri that cost $5 at the swap meet. The shutter closing spring had lost tension when the camera had been dropped so I retensioned it and machined a custom adapter. A simple and cheap camera but I have taken published photos with it.

The electronic cameras sometimes keep you from doing things the old mechanical ones will accomodate. The EOS may require an EOS lens before the mirror will come up and expose the shutter to possible damage. You may have to lock the mirror up or find some other work around...or go buy a $5 swapmeet camera.

Luck,
Rick