Interesting. I always understood that you could get a sundog (or
moondog, for that matter) ANYTIME there was a major occluded front with
enough temperature differentiation - and the sun at the right angle, of
course (even well below the horizon). The interface between the two
different atmospheric densities produce a mirror (as does any similar
sharp density differential - such as between air and water), reflecting
the image of the sun or moon. However, the source you give seems to
credit ONLY the plate ice crystals for this (and for most everything
else G).

jmk