Probably with sql dict too, but I can't remember right now how exactly.
This works for us:
dict {
quotadict = mysql:/etc/dovecot/dovecot-dict-quota.conf
}
plugin {
quota = dict:INBOX::proxy::quotadict
quota2 = dict:shared:%d:ns=shared.:proxy::quotadict
}
in the mysql-table, username and domain are stored in the same column
