There are probably a few people that may fish some of the backwaters but not that many. Access is usually limited to walking in from public access points so you may have to hike in a good distance. Most of those that do fish those areas have developed their spots on their own and don’t advertise their success. There are some general areas that get fished like near Osceola Landing. A few years ago, during a really cold winter period, I fished upstream out of the Franconia Landing a few times. I didn’t have that much luck and I worked hard for what I got.
I would caution you about fishing some of those upstream backwaters – the St Croix River upstream of Stillwater is not at all like the big water from Stillwater to Hudson to Afton and south. Upstream it is a river and a lot of those backwater areas have current moving through them and the ice can be hazardous.
I would recommend a couple of things if you do get adventurous and want to try fishing that area. Don’t go alone; be prepared to possibly fall through the ice; have a plan to get out and get shelter if it is a cold day.