Sounds like Total BS to me.
Hair cards need a lot to work correctly, including a flow map which is predicated on a UV of the whole set unwrapped within it.
Yea, in theory you could sort of do it with vertex colors. In practice you need at least one vertex color channel to do other stuff, so you can’t.
On the other hand, grooms have never worked right since release…
Also, to get from a texture to a card in an “automated” process of any sort, you would need to shoot and bake the hair render onto a texture - the engine doesn’t write files to disk that way, so pretty sure it wouldn’t work.
You’d have to manually save out the result of a render target or have a pyton powered script do so.
All in, I have always suggested that evereone should learn how to make their own hair cards…