if a person dissociates during an EEG it will either register

the person has fallen partially asleep (slowed electrical impulses, slowed brain activity, different parts of the brain activated as if in REM sleep state) which will get you diagnosed with a sleep disorder or seizure disorder.

or

that the person has a seizure disorder or medical health condition (Alzheimer\dementia\parkensons, epilepsy/ MS and other medical conditions that the EEG looks for.

as for the videos posted they are now in the midst of controversy as to whether they are real or faked. the premise is that anyone who knows how to regulate their breathing and focus their thoughts can manipulate EEG's. the videos under scrutiny is because the patients have a background of perpetrating mental illness fraud and as part of their treatment learned self help tools like biofeedback.

to show how that is possible I have been using breathing and focusing my thoughts so when doing my EEG's the test includes the technitions having me do things (open your eyes, close your eyes, take a deep breath, one time they even asked me what color under clothing I had on as a distraction when the test was showing more intentional breathing \focusing activity) even now that all my alters are one whole person with me I can if not distracted through the EEG's off and end up being mis diagnosed.