I really couldn't find much in the way of people using these vertically other than 1UP telling me I should use the roof rack version if I wanted to mount vertical. Why? No clue. Seems like the normal rack but with a T-slot mount. I just happened to stumble on these guys (Owl Vans) that already came up with something. "Proven" and winds up being cheaper than if I came up with my own. Only bummer is that Owl won't change out their hardware, so I'll have to source that on my own as I'm not fastening to one of their systems. Not a big deal.
While I can't speak for their methodology, I was working on something similar for my application. My thought is if the cable is fastened in a way that leaves the bottom bracket slightly acute from 90° with the mounting point, this now becomes a "fixed" position when the bike is installed. This is beneficial in that it cannot extend further for security and also for helping to get the bike put up. Drop the rear wheel in the bottom with the cable holding it in position and crank down on the front wheel with the top, ratcheting bracket. Tension on the front pushes into the fixed rear. Should stay secure.
Bonus points are that it should only add about 3" or so to the overall length of the truck when folded up without the bike canceling the need to remove for general use, and it will be mounting to my tire swing for ultra easy access to the back of the truck for gear and
brews high quality H2O.