Yeah, I'm beginning to think the same thing. Darin agrees as he said above. Store the energy to use all at once. Using it as its produce just isn't enough to do anything meaningful with it.
Talking with Darin, we think that a remote block heater might be semi-possible to do. With 4 hrs @ 36W I'd have 144Wh to play with. That would run a 300W block heater for a half hour. Not sure how much that would raise coolant temps, but its a start.
Edit, I forgot Darin actually did block heater testing. What a swell guy!
So, it looks like 30 minutes on the block heater would get me 10°F. Not really a huge savings, but if I did run the additional battery, I could drain it further without worrying about range. As the car sat during the week it could recharge this aux battery fully. Then, I could probably run it for 1 - 1.5 hrs. THAT would be significant!
edit: I see Darin corrected his images. It is a 30F raise. Not bad.