I haven't heard anything, but my guess is that the ploy here is to suppress the original statement because no lawyer was present at the time. Even though the confession was passed on to the police, it was originally made without legal council, hence, arguably, inadmissible. So, circumvate the confession, go for the insanity plea. It's sick, but it works apparently.
The Fifth Amendment protects a defendant from coerced self-incrimination (e.g., a custodial interrogation by law enforcement officers without the benefit of Miranda warnings).
See ILLINOIS v. PERKINS, 496 U.S. 292 (1990)
(where a suspect does not know that he is conversing with a government agent, pressures that weaken the suspect's will do not exist and there is no violation of the Fifth Amendment).
The Sixth Amendment protects a defendant's right to counsel at all critical stages of a prosecution.
In Illinois v. Perkins, the Supreme Court stated:
"This Court's Sixth Amendment decisions in Massiah v. United States, 377 U.S. 201 (1964), United States v. Henry, 447 U.S. 264 (1980), and Maine v. Moulton, 474 U.S. 159 (1985), also do not avail respondent. We held in those cases that the government may not use an undercover agent to circumvent the Sixth Amendment right to counsel once a suspect has been charged with the crime.
After charges have been filed, the Sixth Amendment prevents the government from interfering with the accused's right to counsel. Moulton, supra, at 176. In the instant case no charges had been filed on the subject of the interrogation, and our Sixth Amendment precedents are not applicable."
I don't think that Hacking was charged with murdering his wife at the time that he confessed to his brothers.
Ruling out violations of the Fifth and Sixth Amendments, that only leaves a possible, but highly unlikely and extremely weak claim for a due process violation under the Fourteenth Amendment.