For voltage sensing, a resistor ladder to bring input voltage down between 0 and 5v could be connected to the internal ADC. Still I would power the entire transponder from battery rather than crossing wires from before and after the ESC BEC.
The problem is that with the 8 pin chip RA3 (that is free) cannot be used as an analog input for the ADC (unless I'm not understanding the datasheet).
For temperature sensing, a 1 wire temp sensor might be used. We need 1 pin to drive the comunication. Maybe it can work in parallel with the LED or maybe we need to remove the led to get temperature reading ... this should be investigated, but also phisical connection from the transponder to the sensor must be investigated.