Why propane? We have a diesel powered Webasto Air Top 2000 unit in our Think City which works just nicely. The same unit is available in gasoline, biodiesel and ethanol versions. It uses between 0.2 and 0.4 liters per hour and generates 1 to 2kW of heat. The difficulty is where to place the fuel filler neck and exhaust pipe. Think solved that by placing the tank and filler assembly behind the rear license plate, which is tilted upwards to get at the filler cap. If you plan to have the tank and filler inside the car I can understand the concern (diesel is messy), but then I think ethanol would be OK.
To get maximum efficiency from a combined A/C and heat pump unit, it needs a computer controlled choke valve that separates the high and low pressure parts of the system, a pump that will work in both directions, and a pump motor drive capable of many power levels in both directions. This is now less expensive than it used to be. Modifying an automotive A/C design to do this might be expensive, but then the Model S is a high end car, the "reduced range in cold weather"-argument shows up quite often, and the Model S will be made in much higher numbers than the roadster, though fewer than the Leaf. I don't know how it all adds up, but I hope for the best.
Toyota had a heat pump in their RAV4 EV, I believe, so Toyota has done it before. Anyway, as so many new electric vehicles are going to hit the roads in the next few years, I would have thought that some supplier would be interested in doing this.
If they end up with a resistive heater, then they really need to offer a fuel-powered unit.
I agree about extracting waste heat from the motor, but I think that that might be a bit challenging. If the battery, pem and motor are on the same cooling circuit they will need to make sure that the heat pump does not negatively affect the temperature of the battery. They might need sensors, multi-way valves and firmware to control the whole thing.