This is not something that I would recommend. Pole spears are not that expensive. Losing one is not such a terrible occurrence that a diver should compromise their safety to avoid it.
What happens when the diver shoots a decent fish with a flopper shaft, the fish wraps the line once around the diver's body and then a shark comes in and grabs the fish or a jewfish grabs the fish and takes off back into his hole? You have a 3-400 lb predator dragging the diver, possibly into a reef or wreck.... if the coil line gets wrapped around a few body parts or pieces of equipment, the 30-lb break away clip will be irrelevant.
Clipping lines off of a diver's harness is not something that should be done without a careful analysis of what can go wrong and what problem you are trying to solve. This device would fail on my personal risk/reward analysis.