Suppose a black hole is formed at time $t_0$, and after that even more energy falls in, in what we are calling mass shells. I'm inclined to believe that the initial black hole starts radiating before it continues growing, and that each shell of mass falling does change that pre-Hawking radiation. And I say pre-hawking because it's not needed to assume thermal radiation is coming out. However, it's not clear to me if there is a way to locate the source of the radiation, i.e. how much energy is drawn from each mass shell.
I may rephase the question by asking this: Is there a way to know how much of the radiated energy comes from each of the different mass shells as sources of pre-Hawking radiation?