authid translation using SASL sql auxprop

[Michael Ulitskiy]

Hello,

Is there a way in cyrus/sasl to transparently change user authid according to result of some sql query?
I.e. I want that if user successfully authenticates as user 'john' to transparently change his authid to user 
'jack' and so let him see user.jack as his INBOX.
After initial reading of documentation I thought I could something like the following:

sasl_sql_select: SELECT password as userPassword, mailbox as authid FROM emails WHERE username='%u' and domain='%r'

I can do all kind of username/domain translation within sql domain (views/stored procedures/etc) so there's no problem to
authenticate someone as someone else there, but how can I change the authid? Can it be done with Cyrus/SASL?
Thanks,

Michael