I tend to agree with the general sense of the responses you have gotten from folks here on the RISA list, in terms of the historical interpretation of these texts. Affirmations of individual quality over birth, while present, seem to nonetheless occur in a larger context that takes birth for granted as a necessary prerequisite for brahmin status.
My own interest is more in how these texts are and could yet be interpreted by contemporary Hindu progressives seeking an authoritative textual basis for challenging (and even rejecting) the notion of birth-caste in favor of an interpretation of terms like “brahmin” as referring more universally to anyone who meets certain prerequisites of character: to put it in more traditional terminology, of guna vs. jati. Anthropological observations, such as those made by Charles R. Brooks in ‘The Hare Krishnas in India,’ and my own observations both in India and in the US, suggest that, regardless of whether one regards such an interpretation of these texts as historically valuable or valid, such an interpretation is operative in the community in the acceptance of people such as American and European Krishna devotees as proper brahmins in certain ritual contexts. One could argue that such an understanding is idiosyncratic. But it is nonetheless a real phenomenon. (I’ve myself experienced it firsthand.)
Arti Dhand (Department and Centre for the Study of Religion, University of Toronto):
Like the others, I also want to caution against reading too much into isolated narratives that question the salience of birth as the primary criterion for caste. It's pretty clear in the epics at least that we have a hierarchical structure that takes it for granted that class status is assumed at birth. While that norm is occasionally questioned, it's not exactly rigorously challenged or even seriously debated.
Having said that, if the material interests you, there are other places you might look more fruitfully for such cogitations. For example, I like the Snake episode in the Aranyakaparva (III.175-178) where the question is raised directly. You know the story:
Bhima is inexplicably trapped in the coils of a boa constrictor, but the snake is no ordinary one, being of an unusually contemplative bent. He agrees to release Bhima if Bhima can satisfy his curiosity about some dharmic questions, but of course this task is too much for Bhima's talents. So when Yudhisthira appears and appeals to the snake, the snake addresses his questions to the Dharmaraja. The very first question the snake asks is "Who is a brahmin and how might we know him?"
Yudhisthira responds describing certain ethical qualities, to which the snake argues: "Authority, truth, and the Brahman extend to all four classes: even sudras may be truthful, liberal, tolerant, mild, nonviolent and compassionate."
When Yudhisthira concurs, the snake objects that: "If you judge a brahmin by his conduct, then birth has no meaning..."
Yudhisthira argues that birth is difficult to ascertain with certainty, and therefore the ethical qualities are more important.
The snake is satisfied by this answer, and concludes: “Truthfulness, self-control, austerity, discipline, noninjuriousness and charity are people’s means to greatness, and not birth or family."
He releases Bhima and it then emerges that this is one of the several tests to which Yudhisthira is subjected--to test his dharmic mettle, so to speak.
It's an interesting passage that suggests that there was at least some reflection on these questions, even if the overwhelming weight of the epic is on the side of orthodoxy.