I did the exact same thing with an Emerald, the downside to the ECU is that the lambda input expects a narrowband signal.
Just decide upon what type of signal you want to send, you could easily use a 0-5 Volt wideband signal and connect it to a free analog input instead of the lambda input. I tried both and found the 0-5 Volt signal more usefule because the car was a rally car with a N/A Audi 5 cyl 10 valve engine that had cams that made it impossible to idle at stoich lambda, so there was no point in connecting the signal to the narrowband lambda input.
I had no trouble, didn't use a resistor and lambda values were solid. I didn't do a write-up, I didn't find any part of it difficult.
