Following up, I worked with both Veeam and EMC tech support, and we were not successful getting the old backups from the CIFS only mtree to the Boost mtree. It came down to permissions that were copied over with the fastcopy. We dropped down to bash within the DD, and tried dd_xcacls and good old chmod/chown, to no avail. So now I am removing backup files from Veeam (not disk), re-importing, and then modifying the job to point to the new Boost repository. At least all of the traffic for the new full backup is staying within the proxy servers. From the proxy that is servicing the DD, traffic is minimal, ~300MB/s between proxies and 4-5MB/s between the Boost proxy and the DD.
Now I need to find out which is more efficient, using the Veeam WAN Accelerator for backup copies, or creating a boost repository on the remote DD, but assigning a local boost proxy to it. In theory the boost proxy should do a similar job as the Veeam WAN Accel, maybe better?