I thought I read that anything from dovecot.conf can be overridden in a userdb lookup. Or a passdb lookup with "userdb_" prefix.
But I tried for fun change log_path but it never worked. Is that because logging is special, already started logging before it comes to the passdb/userdb lookups? So are there some dovecot.conf settings that cannot be overridden?
To my understanding only these extra parameters can be tweaked through the userdb/passdb: http://wiki2.dovecot.org/PasswordDatabase/ExtraFields http://wiki2.dovecot.org/UserDatabase/ExtraFields (+ mail and quota_rule)
Quoting from your second link: "It's possible to override settings from dovecot.conf" (quota rules being a common example, yes).
I've successfully overridden a few different dovecot.conf settings, mostly for lda and sieve, but also mail_debug from 10-logging.conf. So it does work to override *some* settings from dovecot.conf -- but there seem to be a few like log_path that are immune to overrides I guess.