The Clathrate angle on brines
One problem with brines and CO2 is that the formation mechanism for Clathrates is more powerful than the freezing-point depression of the brine.
Michael Max described at the GeoMars conference that when clathrates form, they actively displace salt from solution, which precipitates out. I am checking the details of this, and may even need to run my own experiments with CO2, since he was using methane, but it suggests that even brines won't flow on a CO2-rich Mars.