Looking at that I'm certain it is a Kangaroo Carry holster that I've been wearing as a shoulder holster under a button up collared shirt for years now. It is without question the most comfortable means of carry that I've found, and extremely effective at concealing even a full size hand gun. I never thought of using it that way but could certainly understand the leap in your thought process and can see how it could work, obviously. In essence you're using it much like a belly band holster but with a strap to keep it up rather than the elastic cinching it around your torso.
I'm curious if you had to extend the adjustable elastic shoulder strap, or were you able to get the length you needed as it was made?