So I installed the setup. I ran a wire (purple) from the Arduino up to the ECM, and powered it all up (black wire to fuse tap in fuse panel):
I proceeded to take the car on a long cruise, and unfortunately I read out a "PENDING 1702" code, so something isn't correct yet. That's to be expected given this is such an experiment.
I must be missing something (or a few things). I'll need to do a full highway drive with the Arduino logging the TCM signal, and comparing that to what the Arduino is outputting.
I'll post back again when I learn any more about this. I realize this isn't a hot topic, but hopefully people find this interesting!