The air conditioner needs to be able to dump heat overboard, yes - it doesn't necessarily follow that it needs a separate heat exchanger.
With the Model S (and actually the Volt in different ways,) the HVAC is tied in to the battery system heating/cooling. I wouldn't be terribly surprised to see Tesla extend this, and have a single heat transfer zone to the outside (replace the hood?) that uses a liquid coolant which is heated by the battery, motor/power electronics and A/C (most likely in that sequence, I'd guess) when the car is too hot, and shifts heat among the element in cooler weather (the Model S already does some of this, it can use drive motor/inverter waste heat to assist in warming the cabin in winter I believe.)
An auxiliary inlet is an interesting thought - if they go with the whole hood as the transfer surface as I was suggesting, they could add ducts that route air through finned passages along the underside and exhaust near the windshield for high demand and are blocked at lower demands. This would also eliminate the bumper question - they could add fans to pump air through the finned ducts while stationary and stick with a single integrated assembly.
Can it provide enough cooling? That's convective heat transfer modeling you're talking about, and we need some assumptions to answer it. Here's a quick primer on convection with some calculators:
Convective Heat Transfer
Tossing numbers into their calculator and using their chart, if you assume 40C air (~104F, typical really hot day,) 90C for the hood radiator (don't want it actually boiling, this is maybe already too hot since it could hurt someone,) a 3 m^2 hood, and a transfer coefficient of 40 (trying to use the chart, I think this is around the right number for a car driving on the highway (the last number on their chart is about 36, at around 20 m/s - about 45 mph,) I came up with around 6 kW of heat transfer.
That's plenty for drive motor loads - the current Mdoel S and Volt systems are 85-90% efficient, so 6 kW would be equivalent to an average of 40 kW of drive power over the long term. I'm not really sure how much heat dissipation an A/C system needs. The Volt can get over 4 kW of power draw cooling off initially in the summer heat (I suspect the S is similar, but I don't have data,) and since it has to dump all of that power and the cabin thermal load, it can probably overwhelm the capacity during initial cool down.
However, the normal running load is much lower even on a hot day - this is where your cooling ducts come into play again. To handle surge loads like cooling off on a summer day or climbing a mountain while towing something, you'd open the ducts and possibly turn on the fans. During normal operation after cooling off, passive transfer through the aerodynamic shell should be plenty, without getting hot enough to do any harm.
It's actually a pretty clever idea for an EV with their reduced heat transfer needs, especially if they can make the double sided approach with ducts and fans work. Might make the traditional Frunk smaller or harder to access, though.
Walter