Yeah Hans, I think the optimum solution would be to go and see Grumpy!
I've though about the concept of using a crushable media (solder) to determine a measurement.
I've also thought that it would be like trying to measure something on a trampoline.
I have some 1mm diametre solder that I trialled this morning using a bearing to push down on the solder. The imprint of the outer race on the solder is approx 3mm wide, which is enough to measure.
The force required to produce said indentation on the solder may, or may not be replicated by torquing the cover screws
Thing is, the solder diametre needs to be close to the ideal shim thickness so as to not bow the tranny cover when the screws are torqued down.
I think that your setting putty idea has far more merit than the solder because it offers no resistance to the tranny cover and offers a far more accurate real world measurement. (besides a shim plate 'course)
What type of putty, and how one goes about utilizing it is another story.
I realise that the end play required is less than the thickness of a fly shit.
And, I'm trying very hard to stay away from my usual ideal of.....who gives a shit.