I tried it with my Nikon R10 and Bolex SBM today, with rubber eyecup on or taken off.
For the R10, my iPhone 5 shows an image quite small in the middle of the screen, so I found using the photo capturing program is better than the video program since the former allows me to zoom in.
For the Bolex SBM, mine is converted to super 16 format, so my phone doesn't cover the left and right edge of what the viewfinder shows. I guess iPhone's camera is just not wide enough for this.
All these tests were done by putting the phone directly against the viewfinder.
