My metal bodied Mares regs have never let me down in hundreds of cold water dives. In fact, all of my first stages are "unprotected" by the cold water kits. Dive buddies and I regularly use the Proton Metal and Abyss second stages, and none of us have experienced any free flows from icing conditions.
One dive buddy is an ice diving instructor, and he uses unsealed V16 first stages on his doubles while ice diving. He, too, has never had a problem. In his experience as an instructor, he has seen a number of regs begin to free flow while under the ice. According to his recollection, most of them were balanced piston first stages that were unsealed.
I made two dives yesterday in blustery, 22 degrees weather. The water was a brisk 43 degrees. After the first dive, my gear was left outside on a table. Of course, it had a pretty good layer of ice form from the dive. I entered the water for the second dive, and allowed the second stages (Proton Metal) to sit in the water for a minute or so prior to descending. I also didn't notice any ice crystals being "spit" from the second stage during the entire dive.
Greg Barlow
Former Science Editor for Rodale's Scuba Diving Magazine