The ability to discover these GC's is done during topology discovery that essentially does something similar to the following...
1. Find bootstrap servers when DSaccess is first loaded to find 1DC, 1GC and then confirm that a CDC is available based on those servers. If a CDC server is available, then we initiate a full topology discovery after the SA service has started.
2. Full Topology discovery is performed querying the local site and any directly connected sites along with any Site Cost Information.
3. Once a list of servers is found, we then perform suitability checks.
4. After the checks are done, we then have full topology and if logging for topology is turned to Medium, you should see the 2080 event logged in the application log stating the servers that were found and are usable by exchange and outlook clients. This topology information is what DSProxy uses to pick what GC's outlook can be directed to.
If this is not populating any GC's from the sites where the userdomainGC's are, then your AD Site links would need to be modified to allow the correct referral process to occur.
Mike