How to explain meta-consciousness of private thoughts?

[AshvinP]

...are 'private thoughts' also viewed from across a dissociative border, somehow?

I would say No, based on what BK says in the recent youtube course. He refers to these thoughts as endogenous. Before dissociation, the MAL has only endogenous thoughts.

What about thoughts that are triggered by sensory input (impingement from across the boundary). I would venture that such thoughts are endogenous, since they do not originate from the sensory stream.

There is no reason to assume an ontic reality of "private thoughts" or "private field of experience". That is metaphysical dualism, even though some idealists refuse to admit this simple fact. Just like the materialists and dualists, we are creating our own metaphysical problems which we then go about endlessly trying to "solve", perhaps because we like the idea of endless speculation more than the idea of actually finding an answer to any questions of practical significance to our lives. For the same reason, there is no reason to treat exogenous perceptions-conceptions as fundamentally different from endogenous ones. As soon as we shift from explaining these dynamics in terms of our own currently limited organization (which really boils down to a lack of knowledge), and towards an explanation which finds the "private-public" aspect of our thoughts rooted in the structure of Reality itself, we have reified distinction into division and lapsed into metaphysical dualism.

So to the original question, the confusion comes from reifying "alter" into a structure of Reality where "meta-cognition" is only present within the boundaries and "non meta-cognitive" consciousness is everywhere else. Again that is metaphysical dualism. If we are talking about self-awareness, then the only reasonable conclusion under idealism is that it exists for all perspectives of MAL including MAL. If we are talking about abstract representation of experience, then it's reasonable to conclude that an 'aperspectival' intelligence would not need such representations to absorb knowledge from its direct experience. Only intelligences who are limited in space-time require abstractions to capture the meaning of experiences which are fundamentally fluid and always metamorphosing.