This should be undocumented and unsupported. We can likely base this off of the forceSyncSourceCandidate failpoint: https://github.com/mongodb/mongo/blob/4d67dd9561ed019f4a8017ec04a6aa1e82e0d11b/src/mongo/db/repl/topology_coordinator.cpp#L72, but this should guarantee that the node only ever uses the designated node as a sync source and would never use a different node (it's ok to hang or fail, ideally, if that won't work).