I think that you have found a solution to a problem that several of us had a couple of years back. This jib downhaul was originally published in _Small Boat Journal_ under the name "Gerr Downhaul" when that magazine existed and several of us tried it. None of us could make it work. We all rigged it so that the line went inside the jib snaps, and it did indeed jam. I guess that none of us tried it with the line outside the snaps.
It should indeed do the job that a roller furler does, which is to get the jib out of the wind and not pulling. It has the disadvantage that the jib is either up or down--you can not reduce the sail area and continue to sail as you can with a reefing furler like the CDI. A downhaul would probably work better with the working jib or the lapper on a P-19 than it would with the genoa, because of the size of the genoa. A jib downhaul works very well on a P-15, where a furler is really overkill.
Regards
Tom Grimes
P-14 #363 Far Horizon
Muncie, Indiana