if you sit back and contemplate this project, you have a continuos drive toward equaibrium on a never ending basis.
you have the temperature inside of the container.
an interesting point is that the center never cools or heats directly, it is strictly and only the outside the changes temperature.
the skin of your wrap will shed energy in an effort to reach and equilibrium with the environment.
as that cools, the section just inside will see a temperature gradient and the heat will flow.
if you put the container in a cupboard, you can greatly effect the external temperature as the cupboard comes towards equilibrium with the mass.
what you are asking for is not that hard.
you control the energy to the hot plate.
using your PID to keep the fluid at your setpoint temperature.
now, on your second circuit, you monitor the current and voltage of energy being used to keep the fluid at that constant temp.
should be pretty straight forward to have those two loops.
the real question is where to put the temperature sensor that monitors your liquid.