From Eric C., Zwift,
“Tacx have requested you direct all compatibility questions to their support team. The main issue is that we do not have access to the data that is being transmitted by those trainers which is why they are not yet compatible with Zwift.”
This is true, but there are solutions.
Trainer Road supports the Tacx trainers mentioned in this thread. How, Zwift may ask? Fairly simple. They used power data from the TR community, generated with Direct Force Power Meters (DFPM) up a power ramp while riding on unsupported trainers. Pretty simple, take an average of the samples and, voila!
Or easier still, interpolate power data from the work already done by Power Curve Sensor, http://www.powercurvesensor.com/cycling-trainer-power-curves. Here there are Tacx Flow curves for Slope 3 and 7. as well as curves for Blue Matic, Blue Motion, Blue Twist, among others, as well as other brands.
Now, I have to admit, they don’t provide the algorithm for these trainers, but I think the curves provide enough data points to be able to use them for the purpose at hand. A third plot would be better.
The differences between the different generations of Flow, and other Tacx models, has little, if any, bearing on the performance of the brake.
For trainers like the Fortius, legacy Bushido and Genius it would be more difficult because they don’t use a few discreet curves (settings).
A third, inexpensive option under US$100, may be for Zwift buy a Power Curve Sensor kit and do some testing.
Tacx Flow users - in the connect window of Zwift, choose classic trainers, Tacx Blue Motion Setting 4 and set your Flow to Slope +3. Nearly identical curves, within about 1.25%, close enough to call accurate and repeatable.
In summary, if one has an ANT+ DFPM, they would likely connect that to Zwift. But, it is possible to summon help from the community to compile data for unsupported trainers.