Heating a li-ion battery for use in sub-zero temperatures
Not a bad idea.  I disagree with the other comments about using the MCU to control this.  Why waste the quiescent current?  Periodic brief wake ups do have a real effect over time.  Just add some hysteresis to the comparator so that it's not oscillating.  You could consider having the output of the comparator connected to an interrupt pin on the MCU if needed.

Also, it's probably just a drawing mistake, but I noticed that your FET is in backwards.  You should flip it around so that the source goes to ground.

edit: typo