On the A320, at least, one underfloor cargo compartment is able to be heated with cabin air, and if there are dogs or other animals being carried in there it will be switched on.

A baggage hold full of baggage will retain much of the residual heat from when it was loaded, I was informed by another skipper. I've always meant to put my minimum/maximum thermometer in my baggage one trip to test. Of course in December the ambient temperature on the ground may be below zero to start with.

There are significant air vents between the main cabin and the underfloor compartments (which are fully pressurised, a separate matter to heating), following two major DC-10 incidents in the 1970s when an underfloor hatch blew out at altitude and the resulting pressure differential distorted the floor and jammed the control runs.