[CentOS] Large scale Postfix/Cyrus email system for 100,000+ users

I'm trying to set up a large scale email system that supports 100,000+
IMAP accounts.  We have an existing frontend web interface that does a
lookup on a mysql db to figure out which IMAP server to connect to for
each user.  For the email infrastructure we have decided on Postfix
and Cyrus.  We have configured both to use mysql to get the virtual
user information.

Because of the way that the infrastructure is (biz reasons) we are not
doing shared storage, we have numerous IMAP servers that we distribute
accounts across.  As we add more users, we image up a new IMAP server.
 For our business's scaling purposes this was the best plan.

What I am having a problem is how do I get postfix to transfer the
email to the particular IMAP server that the user account is on.  I
know that I need to use lmtp and transport, but all the examples I
have seen show forwarding all email to 1 IMAP server.  I would like
Postfix to do a lookup for each mailbox and determine which IMAP
server to deliver it to.

Anyone have a working example that they could share?  It would be
greatly appreciated.

thanks
-matt