Not a pisstake. I've not seen somebody doing it but I remember people talking about bedding the actions of springers. I have no evidence it's anything other than an old wives' tale.
I've also read of people just using self adhesive draught excluder foam strip.
The Isopon would be hard when set of course. My assumption is its prime purpose was to ensure the metal of the action is in contact with the stock across and along its full length. This should in theory damp vibrations in the action. I speculate that it may also help with unexplained zero shifts reported in (especially) custom wooden stocks on springers.
Looking forward to the results of your Silkaflex experiment.