you probably wont be able to find anything on the internet about it being associated with things like repressed memories, co consciousness (alters sharing what they know) and the like because Hypnagogia isnt a dissociative problem.

it is being half awake and half asleep at the same time. it becomes a mental disorder when it causes emotional problems like hallucinations/ delusions, loss of touch with reality. dissociative disorders like DID have a diagnostic criteria that reality testing remains intact and the problems cant be because of other problems.... in other words in .......most locations....... having the symptom problems of Hypnagogia would .......most likely..... knock out a diagnosis of dissociative disorders...

that said a person can have more than one diagnosis of unrelated problems... for example I have dissociative disorders .....and.....I have Hypnagogia two separate problems that do not intermingle (they have their own set of symptoms, and problems)

because they have to be treated separately or they cancel out each other for diagnostic purposes I'm guessing you probably wont find anything related to things like retrieving repressed memories or developing co consciousness with dissociative alters while being half asleep / half awake and hallucinating and having delusional thoughts (Hypnagogia)

since you think something is trying to come through while you are in that extremely relaxed state of mind that comes with being half awake/ half asleep... heres a thought....

talk with your treatment providers. there are things like guided meditations and hypnosis /self hypnosis where you can during therapy place yourself in a highly restful state of mind similar to being half asleep/ half awake and during this process you can get the answers you are looking for from your alters if this is what is trying to happen.