There are interesting EEG studies that correlate brain waves (alpha, gamma, etc) with various congitive states. There are even studies where playing voltages and currents back to the scalp surface can beneficially affect cognition.
The thing is that these require the placement of electrodes, with
gel, to the scalp surface. That means lots of preparation; to create a consumer technology EEG electrodes should be effortless.
Hair colorists pull clumps of hair through a tinfoil cover then coat them with dye.
I think it is possible to coat hair with a transparent conductive layer that reaches to the scalp. You could even coat adjacent clumps with a transparent insultator coating.
Then the person would just bundle up their hair with normal looking elastic hair ties orr "claw clamps" that had lots of conduction pathways on them, and a computer that could sense which strand groupings went with which EEG local site (multiplexing)
The multiplexed data is then read from the dyed hair.
Now what about hair growth? It appears this would cause rapid insulative gap from the scalp. Fortunately however I read that hair growth comes in stop-start patterns rather than being a continuous activity. So even if 3 mm of surface area were growing (to insulate) at an EEG site another 3 mm at the same site perhaps would not.
You could get an EEG "perm" and then feed your head gamma waves!