Because of difficulty getting a serial console working with the Pro Mini via the Bus Pirate, I decided to switch to an ATmega32U4-based board, the SparkFun Pro Micro. This has the added advantage of no longer requiring a separate programmer, since it has a built-in bootloader.

After an initial SNAFU (accidentally flashing a hex file built for the ATmega168P to the new board) and recovering using the Bus Pirate, I finally was able to flash and test it by plugging in the 10K thermistor.

It works!

Next, RPM sensing, and figuring out how to make that play nicely with the temperature check loop. Then... maybe a buzzer for emergencies?