Well, I'm not sure but I remember seeing a big place that looked really nice. We were in the air in a helicopter ride over the Napali Cliffs but it was near where we took off from. I would guess it was near Hanalei looking at a map.
There's a beauty of a facility up towards Princeville that is a reining facility, plus it does tourist rides (we did it there )....don't know about dressage on Kauai but this place was great! If I can dig up the name I'll update this post with it.