LEC replications that do not work may be actually working at a narrower electrode spacing. I got lucky as I initially tested 0.2 mm; when I tried 0.3 mm, the effect did not show any voltage in my case (Fe on Fe).
If the electrodes do not need to be in direct sight relatively to each other, but any dielectric material placed inbetween will make them work in the same way as I think I'm observing (still not 100% convinced), it should be relatively simple to use much narrower spacings, at least with planar electrodes. The voltage appears to increase at narrower spacings.
If this effect is due to radiation, it must be of a type energetic enough to pass through at least a 0.1mm mica layer and penetrating enough not to be affected appreciably by it.