I had two meetings cancelled today, so I put the spare couple of hours to work for your benefit!
Here's a very simple part that combines the mount from the back of a Cateye light with a GoPro mount.
Here you can see both together (the light and the new part) and also with each clipped on to the Cateye part that slots into the back of a Fizik saddle.
Here you see the two parts inserted into the saddle.
Now seen with a GoPro camera (newer small Session type, but any other would work).
The clearance between the camera and the saddle bag would obviously vary depending on your camera model and the exact mount set up, but this all works.
EDIT: up on Shapeways if you are really keen to have one!
http://shpws.me/JQqV