For testing the performance of the tracker I'd want to leave one in my car. But it contains a rechargable lithium battery and I'd feel much better if it could alert me if the temperature is outside the battery's recommended range.
To make no changes on the PCB I repurposed a UART header I used for debugging to accomodate a TO-92 Microchip MCP9701. This was possible since the UART pins can also be used as ADC inputs on the PIC18F46K22 used in the RN2483.
So I used one pin for reading the voltage from the MCP9701 and the other for turning it on or off. It's fairly low power (6uA or so) so it can safely be powered from a GPIO.
Now every uplink contains temperature data that can trigger an alert.