Just a few thoughts...
Simple proximity detection could be tried, but work that through....there are different ways to implement proximaty detection, but thinking it through, can you discriminate 2 inches from 1.5 inches or 6 inches? That is an important consideration. Additionally, proximity of the bracelet on your wrist to the sensor on your collar is not the same as detecting when you are pulling your hair. Would it be close enough to be of any value - would there be a large number of obnoxious false alarms?
I remember an early behavior modification technique (please note that I am not, have never been, and will never be a clinician) where the technology was wearing a rubber band on your wrist. When you had an intrusive thought, you were supposed to snap the rubber band and the the brief, sharp pain was intended to harmlessly reset your brain, so to speak.
Consider a matrix of IR detectors in the band and the complementary processing. A type of "gesture sensor", if you will. Mounted in a stationary location, you can detect a hand wave. With the proper sensors and processor and programming, you can wave your hand and it can discriminate a lower-left to upper-right wave, or a middle wave left-to-right or right-to-left. Those are available to everybody and relatively cheaply. I think, but I don't know, that they are using something like that, except the gesture sensor is on the wrist and the "wave" is relative to the stationary head. That and an accelerometer. That is what I think might be going on at the sensor end.
Last week (while playing with an LCD and avoiding finishing an actuator project), I went and purchased a little camera for a Jetson Nano. This camera is off-the-shelf and ~$25 - I am sure you can get them cheaper. Running one of their demos (canned but you build it and can modify and so on), I took this picture less than an hour ago:
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That blurry thing in the lower left is the camera. I am holding a remote control up to it and you see what the camera records on the screen. The software accurately identifies what it is (see what is written on the screen), AND, I can move the remote around in several positions and it still accurately identifies it as a remote.
It is an example of deep learning, AI, better known as "classifying things)...cue nsaspook to post the pic of blueberry muffins versus chihuahua puppies
What I am getting at is this: a camera on the wrist band. The targets are pictures, as, or shortly before, hair pulling or thumb sucking or whatever the behavior. Once trained, the software is constantly evaluating the camera output and comparing it to the targets and alerting when a set likelihood of the target is reached. Mind you, it might not discriminate holding your hand to your mouth when coughing from thumb-sucking but it could come mighty close.
Yeah, it is a lot of processing in a small amount of space and with a large power demand....but I am of the mind that proof of concept is a good place to start.