Bottom line: Nancy MacLean is right, broadly, about James Buchanan. She doesn't use the evidence terribly well, but her central conclusion about Buchanan is rock-solid: Buchanan was not at all uneasy about his harnessing of segregationist energy to try to power his anti-New Deal dynamo. Buchanan thought that keeping Blacks down (and, at the option of the whites in the area) segregated was as key a piece of white "freedom" as low taxes and union busting: Daniel Kuehn: James Buchanan, Segregation, Interposition, and Little Rock: "Today in 1957, Gov Faubus used the Arkansas National Guard to prevent the integration of Little Rock Central High School. James Buchanan was in St. Moritz, Switzerland for the Mont Pelerin Society when it happened (Friedman presented his voucher work) http://www.history.com/topics/black-history/central-high-school-integration...
...and letters between Frank Knight and Buchanan imply they may have talked about the unfolding news in Switzerland. Knight was deeply concerned about it, particularly since he was slated to come to UVA when Virginia was going through a similar crisis. Buchanan was not concerned and tried to convince Knight later in the fall (after Eisenhower federalized the National Guard) that there were no problems of that sort in Virginia, and that racism wasn't even the motivating factor for resistance to the Brown decision.
Buchanan indicated to Knight that he thought Eisenhower's actions were completely unproductive. Taken alone, I think this could be read as a tactical disagreement with Eisenhower, but Buchanan went on to promote Arthur Krock of the New York Times as "good on all this", with "this" referring directly to Eisenhower and Little Rock. So what were Krock's views on Eisenhower and Little Rock that Buchanan considered so good? Krock wrote at least fifteen articles between the beginning of the stand-off and the time Buchanan wrote the letter endorsing Faubus's right to use troops to halt the federal integration orders and criticizing Eisenhower's decisions and his dealings with Faubus. Krock argued that the "autocratic way" of the Soviets was be required to enforce Brown, and he spread the theory that the military was secretly ramping up anti-riot training to integrate the South by force. Krock's columns were dismissed by legal experts, including Thurgood Marshall, as thoroughly confused on the law.
By endorsing Krock on the Arkansas standoff, Buchanan was clearly taking Faubus's side, and I think any hope that he just had tactical disagreements with Eisenhower can be dismissed. These were constitutional disagreements. It's probably a stretch to say from this alone that Buchanan embraced interposition, but that was certainly how Faubus presented his powers.
The language of defiance was shifting and non-committal. Massive Resisters turned on a dime against the approach in Jan 1959. People who talked about interposition rarely put much meat on the bones of that constitutional theory, etc. So pinning him to that firmly is a mistake. However, siding with Faubus on constitutional differences with the federal courts who were handing down segregation orders in Arkansas is a key piece of evidence of Buchanan's segregationist bona fides.
Nancy MacLean mentioned this letter to Knight but did not mention Krock or Little Rock. Later drafts of the Magness, Carden and Geloso paper did mention it but they don't really appreciate its importance in my opinion. I see the letter as one of the 3 or 4 key pillars for the case that Buchanan was a moderate segregationist. Other parts of the letter are more in the classical "moderate" mold: he he talks about integration being "worked out gradually and in accordance with local sentiment". The material about Faubus, though, is clearly among the most aggressive things he ever says about segregation.
PS: I should fill out some dates-so things started on Sept. 4th, the week of the MPS meeting in Switzerland. That lingers for weeks until Ike federalizes on the 23rd and the Little Rock Nine were in school by the end of the month (but it was a bad year). Buchanan doesn't write to Knight until Oct. 24th. The letter refers back to prior conversations they had and the main purpose of the letter is to finalize plans for Knight's stay that they'd been working on since March. So Knight is really committed, hence his concerns. Charlottesville is still relatively calm (aside from worrying about the news out of Arkansas). The big drama in Virginia happened in 1956 when the Gray Plan failed and the Massive Resistance laws passed. Arlington's aborted attempt to integrate had long passed. And no orders were being pressed anywhere else yet although things were working through the courts. So 1957-58 school year was sort of the eye of the storm in VA, with MR laws behind them and the closures in Norfolk, Charlottesville, and Warren County coming in summer 1958...