Let me close this!
I've just recently removed the real patch from my postman's "next"
branch, because even that real implementation doesn't work reliable.
I.e., please forget msg135791.  It was true, but on the long run
mutt(1) sometimes sees all, sometimes only some (really nuts), but
most of the time it simply does not see just any box with new mail.
That is, that "plugged-in filesystem" is simply handled as a pendant.

Remarks: because that stdlib MBOX whispered
  "Where Are Tho{u}, Brother"
to me all the {time}, i've done my own, also just recently:

== postman:
  - test: 321 messages (5083760 bytes) [action=hunky-dory]
  = Dispatched 321 tickets to 1 box.
  [69853 refs] real 0m35.538s user 0m6.760s sys 0m0.904s
..
  = Dispatched 1963 tickets to 1 box.
  [93552 refs] real 0m38.860s user 0m8.697s sys 0m0.985s
== stdlib:
  [83010 refs] real 1m3.862s user 0m10.151s sys 0m7.500s
  [93217 refs] real 7m24.958s user 2m0.174s sys 1m35.163s

Was worth it.
Have a good time!