This is what the Red Cross observes about diapering from its 2007 report on the treatment of CIA-held detainees. It’s fragmentary, and it appears not to be used as a technique by itself, but rather in conjunction with sleep-deprivation-by-stress-position. And Marcy, sleuth that she is, finds a reference to an undated, unsigned memo from the CIA’s Office of General Counsel "coordinated with and drafted in substantial part by OLC" that refers to "the use of diapers."
That could be the legal wellspring for the technique. I have some calls and emails out to see what I can confirm. Keep in mind, as Marcy notes, that footnote 27 on page 22 indicates that the memo was attached to some other document delivered to the IG’s office on June 16, 2003. That doesn’t settle the question of when the OLC/OGC memo was written — ambiguity is the whole point of an undated memo! But George Tenet issued guidelines for the use of "prolonged diapering" on January 28, 2003. Which came first: the authorization to use the technique or the legal justification for it? Remember, Condoleezza Rice in July 2002 approved waterboarding as a policy position before the OLC lent the technique its legal imprimatur.
And also, check out the language in that memo. It appears to be the basis for what’s in Tenet’s "standard" and "enhanced" torture categories.