Welcome to the forum, eddyc.
The idea isn't unheard of, and some forum members with big folding farms have mentioned that folding offsets some - even most - of their heating costs during winter. Hopefully those who have had that experience are available for comment.
As for automating things... first you'd want a temperature reading from some sort of external sensor, I'd imagine. No problem as such, see
http://www.pcsensor.com/index.php?_a=pr ... oduct_id=7 for example. The next step would be to get the reading to some easily manageable format like plaintext logfile. Doesn't look overly complicated either. From there on, you'd have to do scripting/programming/tuning of your own, but the 3rd party API (or telnet interface) in FAHClient v7 allows you to unpause/finish individual folding slots.
So, in theory it's doable, but there's no super easy way to do it. Some programming/scripting skill required, AFAICT. I'd try some relatively cheap USB thermometer first (like the one I linked to). See if you can read the temperature into a simple program/script. Then build a system with multiple high-end (thus high wattage) GPUs and unpause/finish individual GPUs (folding slots) based on the measured external temperature. Tune the program/script as needed, of course.
A bit crude, perhaps, but sounds like something
I would be able to pull off (given time and money), so someone more tech savvy could probably come up with a fancier solution.
FYI, some recent(ly resurrected) discussion on multi-GPU setups:
viewtopic.php?f=38&t=9340&start=15#p252971
viewtopic.php?f=38&t=25372
