Stating that you still work your abs when performing a "deadman" with a seat harness is not a refutation to my above statement. There is no way around using your abs while performing a "deadman" with any type of harness.
With a seat harness and lower hook (and 'power transfer to body' point), your abs are resisting a larger mass above the hook point. Think of a sit-up while holding dumbbells at your chest.
With a waist harness and higher hook point (and 'power transfer to body' point), your abs are resisting a larger mass below the hook point (and less above). Think of a leg raise with a dumbbell between your feet.
Both work your abs.