dovecot director redirect some user to always same backend

Hello list

I have a question on how to enforce certain user(s) always to the same backend in a dovecot setup with director. So I changed my passdb-mysql query

SELECT password AS password, username AS destuser, 'Y' as proxy, (CASE username WHEN LIKE 'user' THEN '10.66.99.6' ELSE null END) as host from mailbox WHERE username = '%u' AND active = 1;

But then I saw in the dovecot manual that the director does nothing if the passdb query already returns a host field. So I'm quite unsure whether a host field with value NULL means that the host field is not present and therefore the director should insert it into the result or not.

Thanks

tobi