scarlson 

Hehe, that's great. Yeah, my trusty fifteen-year-old Topeak Road Morph attached to the top tube with a toe strap. A temporary solution, it interfered with my knees, so I got a Lezyne micro floor drive to put in a seat pack.

Starting with the cable. The nub/nipple appeared to be brass or bronze. It is soldered onto a "bight" (a u-shaped crimp) in the cable. You could solder two cables' ends in there, so you wouldn't need a very long cable. I made a reproduction nub myself, but it is hard because it's so small and incorporates a round tube with a square flange so requires turning and milling on a super small scale. I used a very flexible bulk 20-strand 1mm cable I bought on Amazon. This cable is similar to Campagnolo cables from the NR era, lots of strands and super flexible, but smaller in diameter than those Campagnolo cables. The original is definitely stiffer, and may work better, but it is smaller diameter than even a standard bicycle shift cable. You should be able to buy bulk cable like this as well, or use a tandem shift cable or two single-bike shift cables. Some people use a nub with a set screw, instead of soldering on. You could also make one of these and it might even be simpler. I've never tried so I can't comment on the efficacy, but it looks like it might stick a ways up out of the drum on the derailleur and that might be a problem. It might introduce a beneficial eccentricity to the drum (see below) or it might make things worse.

The key to setting up a Le Cyclo is to know that the cable slack changes through the range of the derailleur. This is because as the derailleur slides sideways, it changes distance from the cable guide under the BB. That is, its cable drum is a tiny bit closer to the bottom bracket cable guide when the derailleur is closer to the centerline of the bike. This of course happens when you're in low gear. You can envision a triangle between the BB cable guide and the two extremes of the derailleur's travel. Since the rod should be parallel to the axle, the derailleur is moving farther away from the centerline of the frame, perpendicular to it, the distance to the derailleur is longer the closer it gets to high gear. Because of this, the cable will be a little loose when you're in low gear and a little tight when you're in high gear, and you can't do anything about this. René Herse was so annoyed by this that by the early '50s, he had fabricated an eccentric shifter to compensate for it.

You can, however, use this to give yourself some cable slack to set up the derailleur the way you want. Here I'll explain what I do stepwise.

1. Make sure the two nuts holding the derailleur shaft to the hanger are snug, but loose enough to turn the shaft. Push the derailleur all the way to the stop on the big cog, and adjust the nuts so that the pulley lines up with the big cog. Now rotate the shaft so that the cable is wound around the drum such that it can pull the derailleur the other way all the way to the low gear. This should give you a lot of slack in one end of the cable. Snug up the nuts.
2. Attach the cable to the mounting block in the lever. Leave the screw a little loose. The cable should go once around the lever drum. With the lever still mounted on the frame, move the lever to one extreme (depending on your preference, you might want the lever all the way back or all the way forward. I think Herse used forward as low gear, but I like forward as high gear). Pull the cable as tight as you can get it.
3. Take the lever off and pull the cable a little tighter. Snug up the screw that holds the cable in the block in the lever. Put the lever back on the frame. You should just barely be able to get it on. Try to shift through the range. If the cable seems too tight like it's binding up, take the lever off, loosen the screw, and give yourself a little more slack. Iterate this process until you've got an acceptable compromise in cable tension between low and high gear. It's not gonna be perfect. You're going to find that it's slightly too tight in low gear and slightly too loose in high gear, and that's just the way it is.

I don't run the cable through the spring. Some have had success doing this, but in my experience this forces it through a sharp angle and it grinds on the spring and binds things up. I don't like this. This may be exacerbated because mine is a 5-speed Le Cyclo and these have about an 8mm offset from the cable drum to the spring hook as opposed to the 4-speeds which have about a 3mm offset, and by the 20-strand non-slick cable I chose, but I can't imagine it would be much better even with a traditional slick cable. Drawings and photos of old René Herses and Jack Taylors with which I am familiar generally show the cable running under the spring and not through it. It is true that the spring is shorter in high gear, and this does provide some compensation, but this is only useful if you have a single front chainring. If you have a triple or double, the cable tension gets changed as you change chainrings. I tried it and it was pretty bad.

If you have any questions, I'm happy to help. It's hard to explain this without taking mine apart and taking pictures, but PM me if you want to talk on the phone or something.

+1 for Yojimbo! One of the few businesses that I'm super grateful for existing.