What I want to do is deploy them around the house and control the way air is circulating and in which direction. Some of rooms get warm, some stay cold, I figured I could push the air around without engaging my heating system and make it into an interesting smart project
AC Infinity makes nice stuff, and there are unofficial / community integrations available to get both the Bluetooth and WiFi versions talking to HA with some success. If you search the forums you’ll find some good discussions.
Regarding register fans specifically, I liked the ACI but I needed an extraction fan and all the register fans are boosters that only blow into the room. I opted instead for just two cheap 12v PWM fans, and bolted them to my vent alongside a wemos D1 mini running esphome.
Vent registers from Flair may make more sense for uneven central heating / cooling because they can actually open and close the vent when rooms are already hot or cold enough, but unfortunately they’re very cloud-based and proprietary — two things I generally try to stay away from.
I went a slightly different direction and installed a few motorized inline duct dampers that can accomplish the same thing if you have access to the ducts themselves. They are easier to integrate and are fairly cost effective via Aliexpress paired with a 24v Z-wave relay.