I really appreciate your enthusiasm. It is one of the core concepts and passwords seemed to be a good way to explore my FIFO idea in a practical manner. It might well have been done before, but from preliminary searches I could not find anything on it(not that I searched really hard). Due to its simplicity, I suspect it will lend itself to some pretty efficient implementations.
I would be interested to hear what other kinds of problems you see it being used for.
Not sure exactly what would be another application, but it's a mental model that looks "general", when we have a few prerequisites, that is, not just a solution, but multiple solutions to a given problem (find a password among many hashes in this case), and where solutions could be related in some way so that using the past solutions could restrict the search space.
I would be interested to hear what other kinds of problems you see it being used for.