Question! Without a hard stopper somewhere wouldn't you risk loosing the bar (i.e. getting pushed out of reach, especially if powered (ring furtherst away from body).
The stopper would have to be located either on the bar or below the bar, with some kind of quick release mechanism, to be used in an emergency, to let the bar slide up the power line. I made one that goes below the bar, as I discussed in the thread: "stopper balls, the good, bad and ugly".
Maybe you could experiment with an old Best stopper ball, and thread it on the 2 power lines, below the bar. Maybe, you could put a little hook on the ball, that could engage a "mini-loop", like the North bar uses. Just something to experiment with...Or maybe, you could use the "noose" principle, and have a line that goes from the little hook through a hole in the stopper ball and squeezes the 2 power lines together, so the stopper ball does not slide.