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For several years I have been using a relay connected in series with the A/C switch and activated via an R-C based timer triggered by a switch on the brake pedal. This means the A/C can be on only when the brake switch is closed with a delay before it is switched off after the pedal is released.
The delay - c. 12 seconds - sets a minimum cycle time for the A/C to be on, so preventing rapid cycling of the compressor, and only a brief, light, touch on the brake pedal to activate when the brakes are not otherwise needed to produce the desired rate of deceleration.
A second switch to bypass the relay is required for driving where the brake pedal is not pressed much eg. on motorways.
Like Teoman, I have found that with the A/C off there is a limited mount of heat storage capacity available in the evaporator and comfort would be improved if this could be increased.
Simply increasing the refrigerant capacity won't work as the compressor is required to pump the refrigerant around the system as well as compress it.
Piwoslaw's suggestion of a phase change material around a heat exchanger could be practical. This is used in the evaporator in cars that implement idle stop-start.
Swapping the evaporator for one that includes a PCM would be expensive, even if you could find one that fits. It might be possible to contrive something that attaches to the vent ductwork, whether behind the dash or in front of it. I'm thinking of something like a small diameter tube and shell heat exchanger in a custom housing/shell - like a section of PVC pipe.
The OEMs use a wax/paraffin as the PCM. For an auxiliary heat storage device I'm guessing a melting/freezing point of 10-15 C would be about right, given that it will be downstream of the evaporator.
The heat storage capacity will be fairly small. The evaporators used as OEM seem to claim around 1 minute of additional cooling. This would be useful but not conducive to pre-charging outside of the car.
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