Locking a dwg File, but allowing additions

One of our clients wants to know if it is possible to lock a set of design plans so that they cannot be changed.

The only thing is that the firm who built the plans has to use those plans to then produce a set of "red line drawing" Work-As-Executed plans. This will use the same dwg file, but will only be adding red lines to show changes between the plan and the actual built road.

Is there a way to lock it up to a point, but still enable additions in a certain layer/colour?

Would it work to lock a dwg, and have that dwg referenced into another dwg, and so provide both files to the builder?

"...lock it up to a point,..."

It's either locked or it isn't. If you want to "lock" a drawing go to the website below and page down to a program called DWGguard TNT. Download the zip file and extract its contents. There is a readme file included that provides the necessary instructions.

http://www.cadcorner.ca/lisp.php

Use this free CAD drawing viewer by Informative Graphics to markup your locked drawing without altering it.

http://www.bravaviewer.com/free-autocad-drawing-viewer.htm

Thanks ReMark, the second option could be a good one to ensure that changes don't get made without the designer realising.

Not sure if you'll find any help from here:

http://www.cadtutor.net/forum/showthread.php?t=31924