Resolution:
If there is NAT (Network Address Translation) in effect between the Driver and Engines, Direct Data Transfer will not function, since the base URL provided by the Driver for the Engines (and vice-versa) is set using a local hostname lookup. Once this URL is translated, it is no longer accurate.

If NAT must be used between the Driver and Engines, then either DDT cannot be used, or a shared-mount data directory (NFS or SMB) must be used.