It looks increasingly unlikely that I will get any suggestions to the original question so I am just going to post my own "solution" to the original problem.
Namely I was looking for a backup utility which will play well with HAMMER history. I should have started by searching this list for my own posts. Namely in January of 2013 I asked somewhat related question "Unison and HAMMER history" https://marc.info/?l=dragonfly-users&m=135857302406777&w=2 I remember that after Unison sync the file history was lost. What I didn't remember until today was that Peeter noticed that the same was true with rsync https://marc.info/?l=dragonfly-users&m=135885584004499&w=2 which indeed shares the main algorithm with Unison. He also noticed that using scp or even a cp over NFS (my observation which is fully tested) will play well with HAMMER history. So long story short it would be fairly easily to cook up such a backup tool which will traverse the on my home directory (running OpenBSD) and just cp the files which have changed since the last run. I also tested rdiff-backup if anybody cares and the result is the same as with rsync and unison. Personally I decided to run HAMMER snapshot as a cron job after rsync and in that way preserve the older version of files. By the way DragonFly NFS server is playing very nice with OpenBSD nfs client and I am tempted to move my home directory to DF NAS. History works like a charm. Predrag
