That's my understanding. Inside the ECU I'd expect to find a pull-up resistor, and an input to an ADC. With 20mA available at the Arduino output you can probably override the pull-up without any problem, but what voltage you end up with at the MS input I couldn't guess. Also, I wonder whether there is any smoothing or filtering between the external connector and the ADC. I haven't checked the MS circuit diagram, but off-hand I can't imagine any reason for the designers to have put in a low pass filter there. If there isn't one then you're asking MS to ADC a square wave signal and what value that will return is anybody's guess.
I feel we're possibly talking at cross purposes here. I'm planning to filter the Arduino output before it goes to the MS, so the MS will "see" a nice, smooth signal (even though feeding it a raw PWM seems to work at the moment). Hoping some components arrive today so I can make up a better test board