Ok, I've finally twigged / resolved this...
It looks like the actual directory in question was indeed orphaned from
mailboxes.db - due to an issue with names and spaces.
If you miss-spell the name of the mailbox - 'reconstruct' / 'mbexamine'
et'al just return without doing anything.
This may be by design - but it'd be nice if they come back with "Cannot
file specified mailbox in mailboxes.db" or something - anything logged or
displayed, that at least hints at this going on :)
As it is:
reconstruct user.nonexistant
Just returns immediately, setting return code zero - which was obviously
confusing enough for me to post here.
So, sorry for the other wise noise - and thanks again, to all who replied.
-Kp