A flexible horizontal line is not the way to go with such a short fall distance. You're like going to experience at least a couple of feet of sag in the line during a fall (unless you have extremely high pre-tension, which requires large anchorage forces at the end of the cable).
The best form of fall protection is to eliminate the hazard (who remembers their fall arrest training??). Is it possible to put a 5'-6' high scaffold along the low side of the retaining wall to reduce the fall and eliminate the need for fall arrest? That, or a temporary guardrail would be my first preference. Otherwise, you may need a more rigid tie-off point and self-retracting lanyards.
Keep in mind that most safety regulations require a safety factor of a couple feet (can't remember the exact number off hand) between the low point of the worker and the ground in the event of a fall. During a fall, the D-ring on the harness ends up by your head, so if you're 6' tall and want a 2'-3' factor of safety at the bottom of the fall, that means you can't have any movement in your tie-off point and you can't have a shock absorbing lanyard lanyard. Designing for fall arrest at this height is difficult. You're probably better off with a travel restraint system instead of fall arrest (if scaffold or guardrails aren't an option).
