Treetop trekking places do this. Forget what they're called but the safety line clips into a sort of box with angled wheels that rides along the cable. The wheels are close enough so the cable doesn't slip through, but there is a gap at the top between the pairs of wheels so the brackets holding the cable can pass through. That's maybe a poor description, but it's like a U-shaped bracket with wheels at 45 degree angles at the top of rack side of the U.