Thanks Pierre, that was helpful.
This program is so close to exactly what I need. The missing piece in Mirror mode is a compare of source and target prior to backup (as offered in Second Copy and SyncBack, for example), copying and deleting only the difference between source and target. If my client data stores were small, this would not be a problem, but the data stores tend to run large and can take a very long time to copy.
What gives me pause is that the target files cannot be trusted - maybe they were moved or tampered with. In this case, the logs would not show that a full backup does not exist, even though FBACKUP thinks they do since it relies on the catalog, not an audit of the target at backup time.
Where is my perfect world? :)