adding a wire for my electric choke.

It seems like a bi-metalic thermal cutout switch could be wired in series and placed in the intake choke thermal well. A relay could be used if you would like to avoid loading switched IGN circuit. I did a quick Google and found some typical thermal cutouts here.http://www.electronicsurplus.com/sensors/temp-switch-thermostat
Seems like about 180F would work. Might want to verify well temperature with oven thermometer to get an idea of well temperature with car warmed up. Also verify that choke bi-element opens choke fully when car is warm. If not, it might need the choke heater all the time, or a resistor in parallel with switch. The resistor would reduce the current to provide just enough heat. Then you would be similar to OEM choke controller.