The heated snowmobile garments would work, and they are set up for 12VDC.
When I was looking into this before, I came across this company (
http://www.kustomkomfort.com/index.html)that sells a control unit and wires that you can sew into a garment of your own (maybe a flight jacket and pants you'd keep at the hangar, or maybe a set of thermal underwear to put on under "regular" clothes?) You could do the whole thing for less than $150. There's not a lot of info on their site, but it looks like the whole setup would draw less than 4 amps when it was on full power (probably not all the time), so even with two people it would probably be okay with the 20A available from the Aerovee alternator. My hands tend to get cold, so I'd probably need to wire up a set of gloves as well.
I can't say I look forward to operating my iPad and the radios with gloves on, and the wires/hookup might be a bit of a hassle compared to just turning on cabin heat and being warm everywhere. But it would be simpler and lighter to implement.
There are also 12VDC small electric blankets sold for use in RVs, etc, I saw them at Menards for about $20. That might be useful as a stopgap measure.
Maybe a heated seatpad would be helpful, but it wouldn't be my first choice. The parts that seem to get cold when I fly are my extremities: hands, arms, legs, feet, face, etc.
(I have no commercial connection to the company named above)
Mark