Ordered an EGT sensor! I don't know if it's precise, but as long as it's accurate that's good enough for me.
https://www.amazon.com/Sensors-Digit...816451&sr=8-19
I plan to use it to more finely dial in my part throttle ignition timing. Right now, holding the car at 65mph burns around 1.35 gallons per hour, or ~48mpg. Holding the engine at 2150rpm in neutral burns around 0.8 gallons per hour, which basically gives me an upper limit of 81mpg @ 65mph if the car weighs nothing and has no drivetrain losses or parasitic drag. If my ignition timing is off, adjusting it could measurably improve these low-load situations.
Ideally a bung is welded upstream of the catalyst, as close to the exhaust ports as possible. I planned to just get an adapter and put it in the downstream O2 sensor hole. I don't particularly car if it's getting temperature readings 300F below what's coming out of the engine, so long as I can tell if what I'm changing causes it to rise or fall, all else being equal.