Hi,
After many hours of searching (!) and lots of testing procmail scripts, I found the explanation I was looking for -- something you explained at http://www.dovecot.org/list/dovecot/2008-July/032551.html
That explains it.
Thing is, though, every time I've seen shared mailboxes -- really shared by multiple staff or not -- the preference is in fact for the shared behaviours to also "share" the Seen flags. Typically, someone doesn't want to read an email that someone else has already picked up and started dealing with or responded to. (They'll file it in due course, but the Seen flag is the first indicator that someone's opened and started to deal with it.)
Hacking source code and branching and whatnot isn't easy or done lightly, but I wondered if anything else had come to light in recent years about this issue.
I'd be as happy getting my procmail script to tell Dovecot to update the index based on the flag, but I'm pretty sure that's not possible :)
Any ideas greatly appreciated, thanks.
~ James.