remote file locking VPN alternative

I have various architectual clients that are setting up remote workers that
need to access cad files on the main server.
the problem with autocad is that opening one file can reference another 10
making it up to 10mb of traffic to open one file and
taking about 7min each .
I am using vpn over an adsl connection which is a problem but another faster
connection is not an option due to cost. mail and .docs
work fine.

The best solution for me is to load the whole projects cad files on the
remote workstation and work locally and have the local file lock the server
file using vpn while in use so no one at the office can change it, then on
close sync the file back to the office and like wise in revese from the
office to the remote pc.

is there anyway to do this ?

digitaldunk