Even though it's an older thread, I wanted to shed some light why Dovecot refuses the alias address from Postfix when querying for quotas.
Dovecot's "quota-status" service is supported only in Postfix's smtpd_recipient_restrictions phase. If you'd try to use it later, in the "smtpd_data_restrictions" phase, the service returned with a permissive DUNNO and warned that:
Received policy query from MTA in unexpected state DATA (service can only be used for recipient restrictions).
However, Postfix evaluates aliases only _after_ the recipient_restrictions phase completed. Even technically, Dovecot couldn't receive the resolved e-mail address.
But even in the unsupported DATA state, Postfix doesn't send the resolved alias. During the communication between the two services, Postfix sends the original e-mail address in the "recipient" attribute to Dovecot, and not the resolved alias, although it's now aware of it.
The solution is to do the same alias lookup on Dovecot side, too (as you mentioned you did it). An example userdb SQL that handles it:
userdb sql {
driver = sql
query = SELECT concat("/var/spool/mail/", maildir) AS home,
(5000 + id) AS uid,
(5000 + id) AS gid,
concat(quota, 'B') AS quota_storage_size
FROM mailbox
WHERE active = '1'
AND (username = '%{user}'
OR id IN
(SELECT m2.id
FROM alias
JOIN mailbox m2 ON alias.goto = m2.username
AND alias.active = 1
WHERE alias.address = '%{user}'))
iterate_query = SELECT username FROM mailbox WHERE active = '1'}
Ákos