Ok, so I did a first test charging at -17C. I had the battery hooked up (after it being outside for a few hours) for about an hour, but could only get 2 to 3A of current into the battery. In other words, the internal resistance got so high it was impossible to get power into the battery. After about an hour, I let the battery warm up for a few hours and then charged it fully at 0.5C. I did a 0.2C capacity test an hour after the battery was full and managed full capacity. My guess is that the time I kept the battery on the charger combined with the low power I was able to put in at this temperature prevented any significant damage.
My next step would be to leave the charger on the battery for a longer period of time, however the power supply does not like -17C either and starts acting up. Which brings up an important point: even if you could charge at -17C, would you trust the charge controller, BMS, etc. at those temperatures?