Joining an Ubuntu client to the domain requires creating an object in the OpenLDAP server, and all write operations against the OpenLDAP server must always be done on the DC Master. The OpenLDAP sync structure to other UCS DCs is unidirectional: from the DC Master to all the other servers.
This is different from joining Windows clients as Windows clients join against the Samba AD DC, and all AD DCs have bidirectional sync. When a Windows client joins, the corresponding server objects are created in the site’s Samba DC. From there they’re replicated to all the other Samba DCs which includes one on your UCS DC Master. From there the S4 connector syncs the object from the Samba DC to the OpenLDAP.
So while it would be possible to retrieve certain information from a site DC (most likely a UCS DC Slave), it wouldn’t actually help all that much as DC Master would still have to be contacted for any write operation.