I settled on something similar to this. The outer server (processing requests from the NAS) uses redundant-load-balance to write round-robin across several (currently 5) detail files.
Five detail listeners (one for each detail file) then feed data to their final destinations (remote proxies, SQL databases, etc.).
It turned out a bit neater than having the outer server write a single file that's exploded by a dedicated detail listener into several files that, in turn, each have a detail listener that actually processes the detail. FWIW, it turned out to be impossible to implement that way since detail listeners won't write to a detail files, even if the output file is different from the input file.
Weird, I wonder what hackery is in the server to stop that... But yes you're correct you only need the detail writers in the outer server, not sure why I suggested the chain. Glad it worked out all the same :) Arran