Well, deathbed confessions have been admissable for decades. However, such a confession is not automatically admissable. Its admissability must be adjudicated. It's apparently the same for assailant identification. The "confrontation" clause of the Constitution (like most of the Constitution's clauses) have been qualified in our Nation's 200+ years of history. Just to illustrate, one can be convicted of murder without producing a body. In the extreme, a defendant could argue that the absence of a body precludes the defense from confronting the facts associated with the corpse.