[Dovecot] Maildir "locking"

Hello,

I'm looking for the alternative of qmail's chmod -t (sticky bit on the maildir root) for Dovecot. What I'm trying to achieve with this lock:

• Dovecot lmtp should give back a temporary error (so the email will be deferred and re-delivered later)
• all other Dovecot daemons (pop, imap) should work as usual, but should not alter maildir contents (they can modify their own files, like indexes, logs etc)

What is the best way to do this? If there is no such thing currently, would it be hard to implement the sticky bit checking on the root?

Thanks,