Looking up a definition for 'hanged' I found this.
"Hanged is the past tense and past participle of hang only when used in the sense of “put to death by hanging”. It’s important to remember that 'hanged' has a very specific use. We only use hanged when we are referring to the killing of a human...
Many would agree, but oddly enough, Fowler's (the prime source of information on use of English) defines the two words as interchangeable. Even if 'hanged' is technically correct, in the context of the phrase, I'd go with 'hung, drawn and quartered' simply on the grounds of common usage; you...