director: pass different username to proxy than the one that is used for hashing

[alex]

dovecot 2.3.13

Hi,

I'm looking for a way to make director use a user at domain that is 
returned by the database for hashing but actually send the original 
user at domain in the proxied request. I cannot seem to find a way.

I can change the name used for hashing by just returning a different 
user from the db. but that user is also the one that is send in the 
proxied request.

this is what I have so far:

director_username_hash = %L{user}

# relevant sql:

password_query = SELECT \
     username as user, \ # -> this is used for hashing if destuser is 
not present. = OK
     NULL as password, \
     'y' as proxy, \
     'y' as nopassword, \
    '%n@%d' as destuser \ # -> but as soon as I add this, this is used 
for hashing = !OK
   FROM dovecot_data('%n', '%d')

Anyone know any clever tricks to accomplish this?

Thanks!
Alex

(background: I've inherited a mail setup where mailboxes have multiple 
logins. (those all share the same 'home' for their maildirs). I need all 
of these to go to the same server. Most ideal would be hashing on home 
'%h' instead of %u :)).