[exim] exim + openldap + hubbed domain + virtual domain

Right now, I've got exim set up as a mail hub for one domain. it accepts
mail externally and relays it internally. I'm looking to add mail forwarding
through an LDAP lookup for a second domain that I have for other users.
Basically I want my exim setup to send all mail for domain a.org to the
server pointed to by my internal mx records, while looking up email destined
for people at the b.com domain in the ldap database and redirecting the mail
to them. My users of b.com email addresses will have a php front end to
manage their user name, real email address, and password in the ldap
database.

Has anyone out there ever done this sort of thing? I'm looking for
recommendations on which ldap schema(s) to use and how to configure the exim
drivers to use them.

thanks

jdm