This is not a fully formulated idea, but as I wanted to seperate it as a seperate post (it came up in the comments to the previous post). I highlighed Rashi's statement that Lot celebrated Pesach as an example of Rashi's use of anachronism, and while that example can be explained away perhaps using a thematic reading, it is by no means an isolated occurance in Rashi. Look at one chapter in our parsha - 27:3 Rashi refers to Yitzchak commanding Eisav to use a sharp knife so his meat would not be neveila; 27:9 Rashi refers to Yitzchak eating korban pesach; 27:9 Rashi refers to Rivka's kesuba. I am sure a thorough review of Rashi can bring to light many other examples. To dismiss all these as non-literal takes Rashi out of his role as interpreter of local textual problems. It also simply does not work when we move beyond Rashi, e.g. what is one to make of a question like how the sale of bechora worked when it is a davar she'lo ba l'olam? Aren't we imposing later halachic standards of kinyanim that may not yet have existed onto the text? Nonetheless, some meforshim have no problem raising such a question.
What I would suggest is that Rashi and many achronim approached the text of Chumash and Midrash as a discrete and self-contained system. Limud haTorah to these parshanim is like working on a mathematical equation; kashes of historical context or practical concerns simply do not come into play. Just like 2+2=4 is true in all possible times and places, once halacha declares lo ba l'olam an invalid kinyan it cannot exist **as a unit of text** in a Torah which is true at all times as places. In other words, the issue is not how historically a sale of something which is lo ba l'olam could occur, but rather how Torah as a timelessly true could text contain such a phenomenon.
By way of analogy, logical positivists say that issues of theology or metaphysics are not cognitively meaningful because they cannot be investigated. When one learns gemara, one looks for consistancy within the text; questions like whether "tav l'meisav" is sociologically accurate or how Chazal came to these umdenot are outside the purview of the Bais Medrash. A Mishna is a unit of text, not a historical statement.
Of course, Ramban, Rashbam, and so many others do not read the Chumash in this way. One can undoubtedly dismisses a kashe like "How could Yitchok eat from Eisav's meat without concern for shechitas mumar - hashta behemtam shel tzadikim ein HKB"H mavi takalah al yadam, tzadikim atzaman lo ksh"k" as taking a midrash too literally or being too anachronistic. However, given that such a question was oleh al shulchan shel melachim (see the Yismach Moshe) forces an acknowledgement that some meforshim had a different way of looking at things.
I appreciate the difficulties with this approach raised in the comments, but I don't see another model that would explain what these meforshim are doing. I'm open to ideas - any other suggestions? Final point: even if one rejects their approach to parshanut, the pilpul and of these meforshim is often valuable in a larger context and is an excellent way to stretch one's thinking. Oneg Shabbos for some is a piece of kugel, for others it's a Yismach Moshe, a Parashas Derachim, etc. Not so terrible : )