You couldn't do some type of grape and prune them to not start until they reach a trellis on top of the wall? I have many wild muscadines on my property that the vines snake WAAAAYYYY up in the trees. Whether it's the deer eating them at ground level or they just don't put out until they reach sunshine, the top of the trees, maybe 20-30' up are just full of muscadines. The vines at ground level, some of them are 3-4" in diameter. It's frustrating because all I can eat are the ones that fall on the ground.
If you could get your vine up to the top of the wall and then train it all over a trellis up there, that would provide your screen. 'Course maybe that defeats the purpose and you could just plant the vine up on top of the wall in the first place. Look up the local extension agent for your area and ask them about a fruit plant with a deep root that doesn't spread out too much. They're very knowledgeable about what grows best in your area.
