One critical point I forgot to mention in my previous post about using Duplicity on a ReadyNAS NV+. I'd forgotten that I had installed an updated version of OpenSSH (5.2) which installs an sftp client among other things. Duplicity uses sftp for practically everything. Even when you specify --use-scp, it still expects to find sftp to check file lists etc.
However a sad discovery I made when I started to upload large files was that the upload stalls when uploading to DreamHost. I have no problem copying large files and directories to my MacBook Air on my LAN. And I have no problem of stalling when I copy the same files and directories to DH from my MBA through the same router and ADSL modem.
I've spent days searching the Net trying to find a fix which will work for the NV+. I've tried the 'MTU' fix, the 'pipe to ssh' fix, the '-l 8192' fix, the '-C' fix, the sysctl.conf fix, the 'rsync' fix etc. etc. All make absolutely no difference.
The only difference I can find in the logs of the two machines' uploads is that the MBA has a long series of 'debug2: channel 0: rcvd adjust nnn' lines and the NV+ has only one of those lines and then it stalls.
So for the moment Duplicity is unusable on the NV+ for backing up to DreamHost.