Well, I’m the original poster on this question. I just sucked it up, and bought a v3. I want the flow sensor that badly ($330 worth.)
I looked at the third party flow sensors people were nice enough to post here, but they all cost almost as much as the V3 AND sensor. And then, it’s not clear if the third party sensors with the V2 will deliver the same functionality as the V3 with sensor.
It seems stupid that Rachio did not ALSO provide their same flow sensor, with a wire for V2, at the same time as launching the V3. I get it that our V2 doesn’t have the 900MHz radio, but a wired version would be even simpler to manufacture, and would give many legacy customers a good option.