You don't have to weave the downhaul line inside the luff and hanks.... that's just time consuming.... especially if you have to change a sail on the water. The sail is attached already so there is no benefit to weaving the down haul line ... A small block at the tack fitting.... tie the line to the pulpit when the sail is off (you do remove your headsail, I'm assuming)... After hanking on the sail, tie the small diameter downhaul line to the first hank with a buntline hitch.... the other end of the line is routed back to the cockpit..... If you're single handing... steer the boat with the tiller between your knees, hold the halyard in one hand and the jib downhaul in the other.... then control the drop, steering the boat under the sail so it settles nicely on the fore deck. Once down, the cleated downhaul will keep the sail securely on the fore deck, and allow you to tension the halyard so it won't slap around. You will never have to go forward to handle the sail until removal or change. If you weave the down haul line along the luff.... there will be a lot of friction, and changing the sail will be much more time consuming and tedious.
The largest size line you would need is 3/16.... I have 1/8 in mine... the jib down haul carries very little load... does no hoisting ... its simple purpose is to control the drop and keep the sail securely on deck.
The "Gerr" downhaul system, the second picture of Granger's post, was presented in this forum at least ten years ago.... I rigged my downhaul that way and was quite disappointed in it's practicality. The downhaul is routed through a mid luff ring, back to the clew, then forward to the tack before running all the way back to the cockpit. The premise is that keeping the halyard cleated, releasing the sheet, and hauling on the downhaul allows the crew to "clew up" the sail for a temporary reef.... i.e. the clew is drawn up to the mid luff ring.... just like on a square rigged sail. If you keep the sheet cleated, and release the halyard, the downhaul will draw the sail down to the deck.... theoretically bringing the mid luff ring with it.... ANYHOW... it really doesn't work like that.... and there is a lot of friction with rings and clew grommets for the downhaul to thread through....The sail gets bunched up and the downhaul line gets tangled in it. I experimented with it quite extensively and decided that my boat was simply too big (27 ft) even with the blade (110) hanked on..... so I removed the gear and went with the simple down haul for the purpose mentioned above.... It has become an important part of my sail handling system.... especially when single handing.
The system might work on a smaller boat... under 20 ft perhaps... or something like a West Wight Potter... but not on a mid size or larger vessel.