Honestly I don't know exactly the Openldap solution,
But you know your requirements.
I have some cases where my customers have different LDAP servers (Microsoft LDAP), located in different domains and we need to query those server with the same time. I was browsing the web if I can find any application that allow to query different LDAP server so I can use it as a work around.
back-meta seems promising, see man 5 slapd-meta
Ciao, Michael.