Ok. I figured it out. If you use the adddiffs command, the filespec in the diff can contain the version. A GNU unified diff file starts out with:
--- from-file from-file-modification-time
+++ to-file to-file-modification-time
I had tried changing the modification-time to be a version number with emacs, but I had convert tabs to space enabled. That space between the file and the modification-time must be a tab character. Now that I've figured that out, I can change that modification-time to whatever I want, and it shows up in the before: and after: drop down boxes in the source view of the review.
