Earlier in the thread, when dealing with the LSU4 sensor. It was stated that in order to read resistance of the sensor, the sensor had to be either effectively disconnected from the ECU in so far as its abilty to be a lambda sensor is concerned. Or, using a high frequency wave measure the resistance by a drop in amplitude.
I don't think its going to be a simple as connecting up some channels to a few wires, while the ECU has power, and obtaining useful readings for calculating the resistance. The readings are going to be infulenced more by sensor activity than anything else.
Have you tried using an IR temp gun downstream/upstream of CAT on a known good car, and comparing to ECU/thermocouple value?