UPDATE: find the video version at the bottom of this post.
Turns out that the oldest 12 magazines (6x6) for the Hasselblad 500 have a distinct advantage over the later automatic ones. You can start the roll a little earlier to fit a 13th picture on the end of the roll. Most of the time...
How to get the extra frame
The paper backing of HP5 120 film is numbered so that on really old cameras you can visually set the first frame starting point after loading. The trick on the Hasselblad is to start the roll on the first circle symbol rather than on the number 1 that you would usually aim for. Other films will have different symbol conventions on the paper, but should work in a similar way.
Another way to figure out where to start is to look at the paper of a used roll. By leaving the tape on the film side of the paper you can visually inspect where the film starts and the corresponding symbol needed on the reverse of the paper. Remember that the symbol is referenced to the center of the first 6x6 frame, not the top of it. The starting point, therefore, is 3cm further into the roll from the tape mark.
After taking the 12th picture, the frame counter needs to be reset to prevent the camera locking the shutter button.
What could go wrong?
Firstly you must avoid starting the roll too soon. This might take some trial and error for your magazine, but if you start the roll too early the first frame might expose over the tape that attaches the film to the backing paper.
The camera is not designed to take the 13th frame, otherwise this would just be the normal number to expect. I often find there is a large gap between frames 12 and 13 - sometimes so large that the top of the last image is lost over the edge of the roll. I'm not sure why this happens because sometimes I get the entire 13th image without issue.
Sometimes the bonus 13th square frame is cropped at the end of the roll of film.
Given the risk, the 13th frame should be seen as a bonus and not used for any critical shots. It is nice to get the most out of a roll of 120 film and the old magazines are the most desirable in my opinion for this reason.