back_meta and back_ldap should forward server controls that clients use in their queries to the corresponding external LDAP servers.
For example, the inhability to send controls to external servers currently causes that RFC2696 paged results control cannot be used, so it is impossible to retrieve more results for a query than the page size configured in the external server, which often is not a big number.
I've patched back-meta in HEAD: if you add the "client-pr" directive, it will perform the client-side of RFC 2696 paged results (either always, with the specified page size, or in response to unsolicited paged results response). This will be totally transparent to the actual client, so the client no longer needs to perform paged results itself.
Please test and report. Thanks, p.