Might be speaking out of turn here and possibly out of my butt, but maybe what I do can help ...
My library sits on drive X: and is backed up nightly using Toucan to drive O: on one of the NAS's, where it is available for use (and misuse) by the rest of the readers here at the Castle of Confusion. THEY use a Toucan script that runs about 45 minutes after mine to backup the O: copy to their local data drives (it varies). All of this occurs at those odd hours of the morning when I'm sleeping. The result is a mirror copy, replete with my updates on their machines each morning and I have backups upon backups upon backups.
Although this does enforce a wait of up to 23 hours for the library to be updated, doesn't it do what you need? Toucan can be set to Mirror mode so that each destination folder becomes a mirror image of the source folder. The initial setup was a bit of a time-taker. But the changes are rare enough (I add books on Saturdays and make edits here and there through the week), that the Toucan script runs fairly quickly. Toucan's a Windows program and is available at
http://portableapps.com/apps/utilities/toucan
One other thing I do in the batch file that Scheduler calls is backup the actual database file. This obviously requires calibre NOT to be running at the time, so adjust the batchfile as necessary in those circumstances.
Code:
@echo off
REM cycle backup of metadata
X:
cd \GaryELib
if exist metadata.db.7 del metadata.db.7
if exist metadata.db.6 ren metadata.db.6 metadata.db.7
if exist metadata.db.5 ren metadata.db.5 metadata.db.6
if exist metadata.db.4 ren metadata.db.4 metadata.db.5
if exist metadata.db.3 ren metadata.db.3 metadata.db.4
if exist metadata.db.2 ren metadata.db.2 metadata.db.3
if exist metadata.db.1 ren metadata.db.1 metadata.db.2
if exist metadata.db copy metadata.db metadata.db.1
E:
cd \apps\backups\ToucanPAF\Toucan.exe --job="xELib2oELib"
In effect, the original data is read-only to the rest of the family because they don't have access to it. They can experiment all they want (and report results) on their private copy of the data, safe in the knowledge nothing they are going to do is going to last past the next morning. You get backups aplenty (I have to admit, Carbonite Pro for 55 bucks a year makes for a secondary level of peaceful sleep) and peace of mind even that the odd accidental delete isn't 'permanent.'
Just a thought run amok, GM