Starting at the end of p. 2 in the referenced thread Engler (2003) said: "The first point I wanted to make [...] was that it takes certain ego capacities just to practice meditation or any spiritual practice. [...] Psychologically, this kind of practice [vipassana] strengthens fundamental ego capacities, particularly the capacities for self-observation and affect tolerance. It also increases the synthetic capacity of the ego. [...] 'Transcending the ego' [...] has no meaning to a psychodynamically oriented therapist for whom 'ego' is a collective term designating the regulatory and integrative functions" (36).
On p. 3 of the thread Engler (2003) goes on to note that some forms of meditation uncover psychodynamic processes but that in itself doesn't facilitate insight into them. The meditative traditions often discourage working with such contents, instead seeing them as manifestations of delusion (43-4). It seems the same is thought of the 'ego' when seen as just an illusion (bathwater), hence little effort was put into its other and non-illusory aspects (baby) necessary for healthy functioning.
In discussing a non-dual state he said "the ego functions as a synthetic principle without organizing experience around a self" (58). (This section also reminds me of Damasio's different selves and Thompson's (Lutz et al, 2007) use of that work.) On 68 he talks about how we access no self via meditation, how we observe the actual process of constituting our self representation from moment to moment. Through this we see the self is not only constructed but requires continual reconstruction from one moment to the next via memory. This process goes 'back' or 'deeper': "The nanas or 'stages of insight' in vipassana practice actually represent progressively earlier stages in the entire sequence of information processing, pattern recognition, and conceptualization by which we bring a self and a representational world into being each instant" (68).
Recall Epstein (1988): "The development of mindfulness...involves a 'therapeutic split in the ego' in which the ego becomes both subject and object, observer and observed. […] Advanced stages of insight meditation involve profound experiences of dissolution and fragmentation, yet the practitioner, through the practice of 'making present,' is able to withstand these psychic pressures. It is the ego, primarily through its synthetic function, that permits integration of the experience of disintegration. In true egolessness, there could be only disintegration, and such a state would manifest as psychosis. […] Thus, mindfulness is not a means of forgetting the ego; it is a method of using the ego to observe its own manifestations" (66 - 67).
Wilber (2006, p. 22) also asserts that it is the self-system (aka rational ego) that integrates all of the various aspects of psyche. And that a strong, healthy ego is prerequisite to take such a journey into transpersonal nonduality, lest the trip be into psychotic dissociation. But again, Wilber is a mixed bag here, often framing such transpersonal integration within traditional views and their own confusions, particularly with reference to states.
To clarify, according to Cook-Greuter (2013) the ego itself has stages. The synthetic ego function appears in all of its stages. My guess is that it takes at least a formal operational cognitive level per Piaget, which is closely correlated to Cook-Greuter's conscientious (or achiever) ego stage. This is where the abstract ego stabilizes and can take a 3rd person perspective on itself, which is in both Engler and Epstein's descriptions above. I claim that this is the witness of meditative awareness, hence it is historically that such traditions emerged when this stage of ego development was also emerging. It might even have been Cook-Greuter's self-conscious (expert) stage, when the 3rd person perspective first appears.
And of course there are some who go post and trans in both lines, but that is a very nascent development and at this point there is much legitimate debate as to its meaning and definition. Hence places like IPS forum where we explore and iron out those details. We're still infants in this process. That's why I agree with some researchers that the so-called postformal cognitive developments like systemic, meta-systemic, paradigmatic and cross-paradigmatic might be more lateral extensions of formal logic so place them laterally on the Wilber-Combs lattice where the states are usually placed. And the states are then placed above the formop level, not as higher cognitive stages but as the folded and consciously integrated earlier, preconscious levels.