It's my opinion that heat sinks or sources on the insulator on the spark plug wire side would have minimal effect on the temperature of the electrode tip.  The proof, of course, would be to try it and see.  I would encourage you to do so.

 

I was encouraged by the statement in the link I provided that the temp difference in a hot and cold plug was 10*C to 20*C.  My supposition (hard to even call it a hypothesis at this point), is that those experiencing SAG are riding on the ragged edge.  Pulling just a little more heat out of the plug would be enough to make them last dependably.

How about this for an experimental setup.  Fit the engine leading plugs with spark-plug CHT probes.  Fashion a heat sink for just one of the LE plugs.  Monitor the temps, and cut the plugs open after 25hrs, looking for the deterioration you noted in the first installment.

Sounds like an experiment to run, just as soon as I start flying.