"The Big Bang theory does preclude a "past" prior to the Big Bang."
No it doesn't. And you went right ahead and showed how there could have been. "There may be something *outside* of our universe." I didn't specify inside or outside of this universe. I just said a "past" prior to the Big Bang.
T-1 ( time minus one ) refers to a "time" prior to the big bang, and what was, may have been, coud have been, there, prior to the Big Bang. I doubt anyone thinks of it as being within this universe, obviously, when discussing the Big Bang theory.
BSB