[EricH]

Look below the ball return in the official drawings. There's another bar there, below the ball return. If you can avoid hitting ball return elements while hanging on that, you can hang on any side of the tower.

As for not being able to hang on the sides, I can think of two robots that would be able to do that right off the top of my head, if they're still running: the 60/254 collaborative twins. They wouldn't balance on the bump, either.

(Now that I've said this, watch those designs be copied all over the place...)