The traveler track is sort of the idea I had in mind for encasing the join. Perhaps a crimped wire loop would serve as how the dog chain was attached to the run, and it pulls inside the track as the dog goes past it. I'd only use that on the section of run where it needs to be supported at Point B, and where there's a directional change.

I've also thought of a gated, spring loaded toggling bracket, but it needs to be spring loaded BOTH ways, because the dog would go back and forth. I'd envision it as V-shaped, so as when the dog went past, it would flick down, and the V would ensnare the wire back into it. The base of the V, or where the hinge is, would be the part thats supported.

I actually did also think of running the wire around the back of the tree, sitting inside a C-shaped steel tubing. That tubing is fixed to the tree, allowing the dog to run around the back of the tree at Point B, but my dog's an idiot and wouldn't go the way i planned.