re: What do you think of this idea for a short hedge?

It will definitely work for your application. Obviously, the more vines you start with the sooner it’ll happen. I used 6 one gallon plants for a 36’ (8’ height) section of fence in my yard and only took 2 seasons (New Orleans) to completely cover it. Used screw eyes and 18 gauge steel wire to train it. In season 4 now and just had to cut it back by a foot+ after bloom and already looks lush again. Just make sure you properly prep the planting area and use some slow release granular fertilizer. Good luck