The simplest non-real time solution (provided you can be sure that only one person is going to be using the central database location is to create a simple wrapper script for launching calibre with rsync)
Code:
#!/bin/sh
rsync myserver.net:/path/lo/library/folder /path/to/local/copy
calibre
rsync /path/to/local/copy myserver.net:/path/lo/library/folder
rsync will only copy changed files and it can use ssh as well as various other transport protocols.