Hi All - Thanks for weighing in. Here's what I did and how it worked out.
Day 1: -82 degrees. F55 was wrapped in a Portabrace polar bag. We stuffed a number of extra large hand warmers in the interior pockets, and around the LCD. Instead of our usual ENG lens, we used a simple Sony e-mount 16-105 (we figured better to kill that lens than any other in our kit). It was a little short to control from inside the Portabrace bag, and I hard a time getting to the wide end of the zoom because the drawstring around the lens kept getting in the way of the mechanism.
It operated perfectly well at that low temp. I shot with it for about 15 minutes. However, as predicted, we got some serious condensation in the lens upon bringing it back into room temperature. Camera wasn't useable the rest of the day.
Day 2: -40 degrees. Same Portabrace procedure. This time after using it in the cold, we dropped it into a standard Portabrace bag with about 6 other hand warmers in there and left it in the cold, powered on. I was able to come and go, using it all day long (1 battery change), with no problem except a little frost on the LCD display loop once (easily scraped off).
At the end of the day, We made a vapor barrier liner by double bagging the camera with garbage bags (in the Portabrace polar bag), and then brought it to room temp. We left it untouched overnight. The next morning, we opened the garbage bags and the Portabrace polar bag. The camera and its bagged environment were completely dry. No condensation at all. It powered up fine, and all the footage looks great. I suggest this approach for anybody filming in really cold conditions.