A manual process is better than no process. However, this is stone-age. DOS solved the issue in the late 80's as far as I remember. People will forget to manually lock files and to unlock them. File locking mechanisms should be automatic at least in MS Office applications.
How to achieve this behaviour?
Alternatively, when will this functionality be available?
Additionally, why not overlay the Dropbox mechanism for file locking with the Windows mechanism, allowing for file locking on a device basis?