I am ignorant of version control systems but isn't what you want to achieve similar? A computer programmer makes a change to the code and the vcs records that and graphically shows the history of all edits, even by different users. Probably the algorithms for doing this are in some free library and you can call them from your scripts if you don't want to force the users to use a specific environment.

Creating a specific environment in which edits are made is another approach if your setup permits: you provide the editor environment within which these changes are made by the reviewer. Because all the changes are made within this editor, it will know what changed, when, by whom and to what, marking these appropriately.

The vcs does not necessarily need fresh texts. If you have the progression of a text: T0->T1->T2 then it can probably still work as it does not. I think, tracks user keystrokes but it diffs text as you want.

bw, bliako