Suggestion: Use Volume Snapshot Service (VSS) to sync locked files.
A folder sync tool with encryption
Brought to you by:
steveking
Originally created by: saku...@gmail.com
There is a example here: http://www.duplicati.com/
"Duplicati can make proper backups of opened or locked files using the Volume Snapshot Service (VSS) under Windows or the Logical Volume Manager (LVM) under Linux. This allows Duplicati to back up the Microsoft Outlook PST file while Outlook is running."
There are many backup and syncing solutions use VSS to read locked files, windows' built-in backup solution to hot backup the whole system volume(C:\) is using VSS too.