rewrite searchDN based on filter

I know this was discussed recently but there didn't seem to be a real answer so I'd like to kick if off again.

I would like to be able redirect user searches to two different LDAP servers depending on UID, so:

if a process searches for userA with a base of o=container, then the request is sent to ldap://10.0.0.1/ou=subA,o=container
if a process searches for userB with a base of o=container, then the request is sent to ldap://10.0.0.2/ou=subB,o=container

I'd appreciate any information relating to weather I should be using the ldap or meta backend to achieve this. Mostly I'm having trouble understanding the best way to rewrite the searchDN based on the contents of the searchFilter.

Cheers,
Finn.