Restating the question and the points that have been made to help myself think through the scenario - we have three OCPDs in series - a CSR main breaker, a fuse, and a BR branch breaker. If the fuse weren't there, the series rating says that the CSR main breaker would trip (clearing the fault) and that the BR branch breaker would be able to withstand the let-through current (without being catastrophically destroyed). So the question at hand is - Could adding a fuse between two series rated breakers slow down the trip behavior of the main breaker, thereby increasing the amount of let-through current beyond what the downstream breaker can safely withstand?
I think that the (perhaps unsatisfying) conclusion we're coming to is: Maybe - predicting fault behavior is really hard, which is why series ratings rely on testing and listing between very specific combinations of equipment. The safe answer is that the fuse will affect the fault behavior and series rating between the two breakers can no longer be applied.
Thanks everyone for the discussion! Feel free to continue it, but this will probably be my last response.
(Am I supposed to mark the discussion as "closed" or "answered" in some way?)