Switching a sensor in or out (between 2 microcontrollers)
If you can figure out how the existing controller reads the sensor, then maybe you can just watch the voltage, and the existing controller would never know.

I know you said the existing controller is undocumented, but there aren't that many ways to read a resistance.  You have a reasonable chance to figure out what it is doing.  Use a 1.5 k&Omega; potentiometer, or even a switchable resistance box.  Most likely, the circuit either uses the RTD as one resistor of a divider, or it puts a fixed current through it, and measures the voltage.  There are other more exotic ways that might be difficult to work with, but you have a good chance to be able to figure this out.