The v2 has a WiFi radio. From the other posts I’ve read on the flow meter, it uses WiFi to transmit the data. Having an 802.11 radio is all you need to be compatible. It’s a software solution, that should be doable in the v2. The only reason I would see for the lack of compatibility is processing power. If that is the case, I would like to know that from Rachio. What is the reason for the limitation?
The concept behind this seems to be marketing. Asking people to upgrade to the new controller so they can sell more controllers. Not acceptable. And if this is the case, I will no longer recommend it to anyone who asks.
EDIT: As a side note, they designed the flow meter, why not design it to work with the v1 and v2 models? Design it to use an 802.11 radio.