Once you install the Jtek, it is cable tension that keeps the cable in place in the sheaves of the pulley that it wraps twice around. Once you split the cable on a take-apart bike by unscrewing the two ends of the cable-joiner, the loss of tension causes the cable to unship off the pulleys, going haywire as it were. When you reassemble the bicycle, you have to bed the cable back into the sheaves and orient the pulleys properly...and you have to maintain hand tension on the cable while you find the other end of the joiner and screw them together. If you lose your grip on the rear half of the cable before making the connection you have to start all over again.

So what you want is some way to prevent the cable from straightening itself out of the pulleys when there is no tension on it.

Rummage through your loose-screws drawer until you find a small flat- or button- head bolt and matching nut -- M4 is fine. The ubiquitous M5s are too clunky and bigger than necessary. Shift the rear derailer onto the smallest cog. Then position the nut and bolt around the rear derailer cable two or three millimetres forward of the cable housing stop on the chainstay -- no more. Tighten it gently to hold it in place so it won't fall off. I filed a little groove in the underside of the bolt head to help it grip the cable. A small washer under the nut will keep the nut from trying to eject the cable as you turn it. Trim any excess length of bolt that sticks out past the nut. When you are happy with it, take it apart and reassemble with Loctite. Check to see that the bolt does not rattle or rub against the chainstay -- that's why small bolts are better, the smallest you can work with, really. But with the nut and bolt so close to the cable stop, they can't bounce high enough to hit the stay.

Now when you split the cable, it will back out only that couple of millimetres before the bolt butts up against the cable stop. The cable will stay on the sheaves and you won't have to worry about it when you assemble the bike.

In writing this, it just occurred to me that you could crimp a lead fishing sinker around the cable in the same fashion, although you might recoil emotionally at the idea of putting lead parts on a bicycle...
Or how about cutting the closed end off a cable end cap, threading it onto the cable before you pass it through the housing, then crimping it carefully in place? You would have to do this while installing the cable of course -- not as an afterthought like the nut-and-bolt method. Yet another suggestion would be to super-glue in the same position two or three of those little rubber donuts that are sold to prevent cables from hammering the bottom of the keel tube. Again this is best done at the time of the original installation and setup of the rear cable. (I think I'll try the donut method when it comes time to replace the cable, if I remember!)

Just remember that your cable stopper doesn't have to resist any tension beyond the tendency of the coiled cable to want to straighten itself -- all it has to do is not fall off or be displaced forward or back from where you put it.