I've done some research into stitching together Warcraft 3 replays from before and after a game is saved and resumed. It doesn't seem to work. Anyone interested in more information can read the following topic:

http://forum.codelain.com/index.php?topic=8311.0

You will find the C++ source code for the project in the GHost++ SVN (revision 318 is current and it is likely the code will be removed in future revisions):

http://code.google.com/p/ghostplusplus/

I just thought I would post this in case someone finds it interesting or decides to try it for themselves. This should serve as a pretty good starting point.

EDIT: It might actually still be possible. I've updated the linked topic with some new ideas.