Recirculate cockpit air to keep feet warm?

The sun and you are about the only sources of natural heat at altitude. Outside air circulating around the feet is especially cold. After eliminating that, then it's conceivable that circulating inside air with the right sun angle might help. But the sun isn't always going to be so cooperating.

For more comfort on a less than nice day, you can be adjusting for +70 on the ground and -30 in the cockpit at altitude. Eventually some heat for the toes is good. With electric heaters under your foot pads, it takes less than a watt per toe and you definitely need to be able to control it.

The best socks I've found are these.
https://www.thorlo.com/shop/men-mountaineering-socks

Those, plus boots, and adjustable layers starting with capilene long johns work for me.