Halcro, To reiterate, copper does not shield against magnetism, electro- or other, largely because, as you noted yourself, copper is non-magnetic. Also, electrical insulators that you list are not good insulators against magnetism. Stainless steel can be magnetized because it has a (low) iron content. Alu will be as good as rubber, i.e., not good. Distance from the source and ferrous materials are two ways to reduce the magnetic field.
Chris, I think you answered your own question re the Dynavector, if I understand you correctly. It seems there was no detectable contribution from the magnet at the rear of the DV pivot when your detector was at the headshell. Thus, I would think, there is nothing to worry about (based on the fact that you read the same value of 40-ish when the tonearm is not in the picture). Moreover, the DV magnet is actually two magnets held in place in apposition over the stabilizer blade that is fixed to the moving part of the tonearm. Most of the magnetic lines of force would stay localized to that area because of the interaction between the two apposed stationary magnets and the moving blade that is already deliberately created. Yes, I think DV knew what they were doing, as they have stuck with this basic design for nearly 30 years.
Same goes for the Verdier: it uses two huge magnets of opposite polarity to elevate the platter. The magnetic lines of force are therefore concentrated in that space between the two giant magnets. Stick your detector down there, and see what that does. Also, the thick platter gives distance from any possible problem up on the platter surface.
