One of the things that bugs me about the DIY electric car crowd is that they're too "green" minded in some areas.
I think it's not a matter of being too 'green'. While the ability to convert a car to electric is a daunting task indeed, it's mostly mechanical and electrical, they don't necessarily have the experience with gas.
Would YOU want to try to home-brew a mobile gas or liquid fuel heater? One that can be activated while in the cab, won't spew CO into the cabin, can be throttled/controlled, etc...?
Tossing a resistive heating element in there isn't efficient, but it is cheap, easy, and relatively safe compared to propane heat.
I think that there have been SOME to do something like that - but it could partially be invention exhaustion - by the time they've made the car, it's too much effort to go that last inch. They just declare it a fair weather car, using a more traditional IC engined vehicle when the weather's foul.
Air conditioning on the other hand is going to suck juice of the batteries. There's no way around that one aside from tossing a generator onto the car, which brings us back to the Volt design.
Well, they could use a condensation cooler, or that one Einstein co-patented. Still, probably better to drive an electric pump with all the electricity you have available anyways.