Downhauls were once pretty common; I agree with Rich P that you will still need to figure out a way to keep the sail aboard. Netting between the lifeline and toe rail will do the trick. You will need a standup block for the headstay, a way to run the
line past the stantions (how about stantion blocks?), and some kind of
cleat in the cockpit area to fix the bitter end of the downhaul
line.
Jib downhauls are not as prevalent as they once were because of the success of roller
furling units RFs are now reliable, standard equipment. They are more expensive than a downhaul, but the prices have come way down over the years. I used a makeshift downhaul on my Oday 23 before I bought a CDI RF unit. After using the RF, I would never go back to the downhaul. It is just so much easier to unroll that sail when I need it; no unpacking, no hanking, no trips to the foredeck in bouncy seas, no unhanking and repacking the sail. Worth every penny I spent.