Not sure how we settle this one, as it's obviously a rare event.
Wouldn't this mean that any rain that has fallen through a sub-zero layer would be hazardous to aircraft below that layer? That's not such an unusual event. It would have to spend sufficient time in the sub-zero layer to become supercooled, which generally takes a while with rain.
I would have thought that the heat capacity of an airframe above zero would melt any such ice in short order.
If we put the airframe in sub-zero air, without enough skin friction to heat it above zero, then obviously we've got a popsicle in the making. But that's conventional freezing rain, isn't it?