After the third official adapter cord destroyed itself, I've taken to repairing them with heat-shrink tubing as well. The tricky part is that unless you want to cut and resolder the cord, you need to get heat-shrink tubing that's large enough to fit over the end connector.

I've found that 3/8" heat shrink fits over the connector, but 1/4" does not. But the 3/8" (at least for the brand I got) does not constrict down enough to fit tightly on the think cable. So I bulk up the cable with 1/4" silicone aquarium air pump tubing.

Instructions: Cut one ~4" piece of silicone tubing, slit lengthwise, and put just below the connector. Cut another ~4" piece of silicone tubing, slit lengthwise, put over the first piece of tubing with the slit on the opposite side. Put this second piece slightly higher, so "top" of tubing covers the base of the connector, and the "bottom" of tubing is ~1/4" shy of the inner layer.

Then cut a piece of 3/8" heat shrink to ~4.5" long. Slide the heat shrink over the connector on to the two layers of silicone tubing, so the "top" is tight to the L of the connector, and the "bottom" extends just past the farther (inner) tubing. Then shrink the tubing with some appropriate heat source (or carefully shrink with an inappropriate source like a match or lighter).

I just emailed this to off to the couple of friends of mine who still use MBP's. This is a real, real clever solution. Read this post, it's very clever not just as an instructive on heat-shrink repair, but the composition (if I'm visualizing this correctly) also functions as strain relief, which is ultimately the cause of fray[1]. My way around the 'over the connector' was just to oversize via teflon tape (naive solution, I know), but your idea is simple yet genius (silicone is soft, so it won't score the existing exterior of the wire, and silicone is rigid enough to act as auxiliary support).

If you're rough on your gear (I've thrown my laptop into a TSA container in a 5:30AM grog more times that I can count), I'd do this as a precautionary measure even if you don't see any damage yet. It might not look pretty, but functionality over form any day.

[1] combined with using thin wire shielding in order to maintain a 'sleek' aesthetic

Thanks. Yes, the goal of the step-down on the cable end is to improve the strain relief. I tried earlier rigid solutions, but found that they soon frayed below the fix. My attempts with tape started out well, but eventually the tape would slip and the heat shrink would slide down. The silicone tubing has more "grab".

I think you are visualizing it correctly. Let's see if I can replicate it with ASCII art (not to scale):

  ------------------------------ 
      ==========================******************
    =========================*********************  
  ++++++++++++++++++++++++++++++*********O********
    =========================*********************
      ==========================******************
  ------------------------------  ****************
                                  ****************

  - layer of heat shrink -------   Rigid Connector
     = outer layer of silicone =******************
   = inner layer of silicone =********************
  + Thin frayed Apple cable ++++***color O led ***
   = inner layer of silicone =********************
     = outer layer of silicone =******************
  - layer of heat shrink -------  ****************
                                  ****************
 

It comes out looking quite nice. The rigidity of the tubing plus shrink wrap isn't a problem if you plug in so that the rigid part is along the side of the laptop. It actually provides a fairly nice handle for unplugging, since you can get good leverage anywhere along the shrink wrapped portion.

Another alternative is to use a blob of Sugru to add better strain relief to the cable entry point. Sugru has just the right amount of flexible stiffness when set. Use the white stuff if you want to hide it, or a nice bright colour to call attention to the fact that you’ve had to fix Apple’s design decisions.