A strap like this could easily be improvised for pretty much any bag system – the Domke one isn’t too expensive – US$27 at B&H and the bags it’s designed to be fitted to already have appropriately located rings.
B&H states that it is designed to fit the F1X, F2, F3X, F4AF, F7, F804, F805, J1, J2 or J3.
It is also quite easy to sling the harness off from one shoulder and swing the bag around so that you can access it normally – the mounting points for the harness are all clear of the bag’s opening so it makes the bag into a somewhat-Slingshot-esque quick access backpack.
All in all it’s a feature of the Domke system which I greatly appreciated with the F-3x. I didn’t end up using it all that frequently as I have camera backpacks for when I know I’m going to be walking very long distances but they’re quite slow to access; using the harness you get a mix of both worlds in a similar way to what Lowepro’s Slingshot line gives you. Not quite as elegant and a little bit more strange looking but quite functional.
Comments, suggestions and criticism welcome!