I received a response 8 hours and 23 minutes after submitting my request.
It shows that the Estrel Reservation system received my request at 07:01 CET.
I didn't get a room. And I didn't make the waiting list either, apparently - "...our waiting list is also completely sold and closed."
Ah, well - I will be staying again at the Andel's Vienna Haus, up on Landsberger Allee...which hasn't really hurt my EF experience much, and has probably encouraged me to see more and experience more of Berlin's culture than I would have if I had just stayed in the vicinity of Neukoln and the Estrel Hotel.