Resolution:
BC supports a property (under Application Management>BusinessConnect>Configuration>Component Settings tab>Intercomponent Advanced) called Skip Content Threshold (KB).  This threshold determines when to use the FCP transport instead of the RV transport between the DMZ and the interior server when handling large payloads between the BC DMZ and the BC interior server.

In the case of EZComm, this applies for both the DMZ to BC and BC to PP.

The other protocols may have  special requirements. If so, each protocol will have its own settings.

In all cases, the BW private process must have to access the "shared directory" and use the same file definition that the BC engines use, as the filename will be passed to BW using the file path and name that BC used to create it.  This is either a directory path using NFS on Unix machines, or a UNC path (//servername/sharename/directory/file) on Windows machines.

How do I configure BC to pass payload content By File Reference from BC to BW?