Yes, this! You speak of Sunyata, that insubstantial aspect of Bodhi as a state of permanence or enlightenment, perennially residing inside of all of the seeming impermanence, and therefore, it remains intact in it's emptiness. This is a definite oxymoron and perhaps it is intended as one? I have always believed that Lord Buddha was putting these ideas out to snare the habitual human propensity to continuously conceptualize about everything. Admittedly, I might be wrong, wrong, wrong.
So, maybe Mahavira, Buddha and Lao Tzu never used words like God, Brahman or Ishvara, because it was just another trap for our conceptual minds? You know, we worship a God so devoutly that we remain separate observers from it's totality and unity, and still exist consciously, as apart from it's unilateral reality. Seeing ourselves as parts of the whole, instead of the interconnectedness of the whole, the Oneness. We perpetuate the subject-object duality in our cosmology and so, project a division of observer-observed, upon that Field of Unified Being, which has no duality at all. This becomes a mess, as history records.
This is not so different that the Hindu belief in an undifferentiated realm of Brahman, which despite remaining wholly unbound and infinite, exists freely within all of the transient forms of all existences. The common ground is the core being, itself insubstantial and transcendent of all passing phases of impermanence.
Atman or self or I, is the most difficult aspect of our being to clearly pinpoint with any modicum of success. Hindu thought describes a seeming dichotomy betwixt Jivatman and Paramatman. I myself echo this idea with my terms, Iso-self and Omniself. Neither exist apart from one another but we use them to refine our intelligence. I find nothing more fascinating or challenging than the idea of duality... and it's ensuing polarities. Lord Buddha, in an effort to point out the mirage of separate existence, chose to emphasize that Atman is also quite an illusion. What individual self? We are never separate parts from the whole.
We do not truly split into two frequencies of self, one temporary and one eternally free of division. He taught that the Iso-self is unreal and that the Omnislef is the only reality. If so, the Omnislef doesn't exist as a separate part, either. It is also a mirage. Anatman is the only permanence, as it never divides itself from the Void. I think he wanted to point out that Self is not ever divided, as it doesn't really exist until the paradox of duality is created. Only in the mind of the unreal, is Self divisional, ego is separate and divided from the non-self. So, it makes perfect sense to propose the notion of Anatman, since self as a dichotomous paradox, is impermanent. Real self, unreal self, non-self. Words... human words.
But when 20th century sages like Sri Ramana Maharshi used Atman as a synonym for God, the Atman is the undifferentiated totality, it adds confusion to the whole mix. I can't say why he chose to ignore the use of the word Anatman, other than he not being a Buddhist and himself, being an Advaitan. "You say, potato, I say potahto. You say tomato, I say tomahto." He stressed that the "I" perceiving from deep within the center of our and everyone else's quintessential being, is that Supreme Self, the Indivisible Oneness. And who can say with any certainty? Most of the highest truths are completely nonverbal and defy description, so we are playing a game with words and ideas.
Great masters of realization do speak, though. And while we cannot speak of the Absolute in relative terms, conceive of Infinity in finite ways or approach an understanding of the Eternal, using our transient values or sequential conceptualizations... it is a fantastic exercise for our mental focus. Our concentration is sharpened each time we go around and around on this merry-go-round. It's what we do, we humans. It's good fun too compare and contrast human ideas, though. This thread has engendered a lot of intriguing stuff, and it is most fascinating to explore. As long as we don't take it too seriously, it's play and a grand time!