I think the answer to the original question is that this heating of the battery is already being done when you have limited regen...how aggressively I am not sure (unless you are routing to a nearby Supercharger), but I'm not 100% sure it is being done in the case of a completely frozen battery.
A few questions about this. Let's put aside the possibility of new hardware and just talk about what the car is currently capable of.
Right now, as I understand it, the car has:
1) A way to run the motor with zero torque production (the rear since it can be done on the RWD vehicle) inefficiently to produce heat and heat the battery via the coolant loop. I've heard this is 2kW heat output capability but not sure whether it has been increased. The energy to produce this heating can come from the battery (it will work when going uphill, so runs in such a way as to produce torque but produce it less efficiently than normal). More questions about other energy sources below.
2) A Cabin PTC heater, ~7kW, which runs directly off the (HV?) battery (I assume it runs off of the HV, and not the 12V DC/DC.)
3) Regen capability, which is easily capable of producing 60kW of power by producing a charging voltage (~400V or whatever voltage is produced for the particular SoC when pushing the available current into the battery) for the battery, but it must be limited if the battery is frozen (to prevent damage). 7-9kW to run the various heaters is not going to produce a lot of slowing: At 25mph, 7kW will produce: (P=Fv, F=ma)
g's = 7kW/(25mph)/4200lbs/((9.81m/s^2)/g) = 0.033g (Compare to emergency braking g's with MXM4s of ~0.95g, ~30 times as much)
Is all of the above correct?
Assuming it is, I have a few questions, in the condition of a 100% frozen cold-soaked battery (let's say it is at -10C), which is absolutely not able to accept any charge:
1) With this 100% frozen battery, for the battery heating produced by the motor, does the car have the capability of using regen to produce battery heating? Or must it extract that energy from the battery only via sending current to the motor? I can't think of a way to empirically determine this. We know there is a way for the car to limit the amount of regen power in cool conditions. But can it produce these low levels of "regen" (it wouldn't really be regen in this case because ZERO current can go to the battery since it is frozen) AND operate the motor in this inefficient warming regime? In other words, is there a way for the car to produce low levels of torque resisting rolling and put the resulting energy on the HV bus, but precisely in such a way as to ensure zero current to the battery (and of course zero current from the battery), run all the accessories, produce heat in the motor for warming the battery, etc. Or in this condition does Tesla just simply prevent any regen, and actually send power from the battery to the freewheeling motor in a way to produce zero torque but produce battery heat?
2) Does turning on and off the CABIN heater in this condition affect the amount of regen available on a hill? To some extent this might answer part of question 1...in this "zero-battery-charging-allowed" frozen state, is it possible to extract energy from the hill (via the motor AC-DC converter), at all? Does anyone know? The empirical test would be (with a very cold cabin as well) to turn on and off cabin heat with a completely frozen battery, and see whether the amount of "motor braking" you get changes. It would need to be relatively flat to be perceivable, because the g forces would be quite limited as mentioned above. It might be possible to see the effect on the power bar/regen bar as well, if the surface were perfectly flat and you were moving at a constant speed, as you toggle heat on/off.
If these questions could be answered we'd have a good idea whether Tesla was optimizing the efficiency of the heating systems with current hardware. I would think they are, though maybe they are gradually tweaking/improving as time goes on, too.
