With winter upcoming, I've been thinking the same thoughts of possibly installing a fuel heater as well.
Although it's for an iMiEV, this video series I thought was very helpful in understanding the issues involved with integrating one of these types of heaters into an EV:
I'll add that the earlier suggestions of pre-heating, an electric blanket/jacket or other device to bring the heat to you instead of heating the air in general is a good idea if you haven't already tried them.
Our Leaf has the cold weather option - the heated steering wheel and seats really are nice. From a comfort perspective, I find that I don't have to run the cabin heater until the outdoor temps are down into the low 20's (F). However, from a safety perspective, there are also plenty of days where the combination of cold air and humidity make heating the air a requirement just to maintain clear windows inside the cabin too.
If you haven't already seen these, another idea to improve range in cold weather might be to look at the various heater mods that allow you to run the blower (helping keep the windows clear) without the heater draining the battery:
If you do take on this project, it would be great to see some pictures!
2012 Leaf SL - 72K miles since 11/2012 - No P3227 Update - 7 Bars / 38.80AHr / SOH 59.15% / Hx 51.26%