You're advocating the use of the well explored EMG (electromyogram) interface. These exist and work but the amount of training to operate one is actually higher than training a person to move themselves. They're used for subvocal speech and no hypnosis is needed. Galvanic skin response is a subset of EMG.

The winner would be a plain old analog interface which you can operate with body movement, not to fine yet not too gross. Keyboard is a similar thing but digital. Mouse is analog but planar.

Getting higher dimensional than 3D + time is very hard (gestures) and that is what's required for speech or fully operating a machine. Thus the projections.