I'm not aware of any scripts for openacs that displays diffs for wiki text. It's on my todo list, and I think others would also like to have this feature in the wiki/xowiki packages. You might want to look at the software that we use for displaying diffs for source code in the cvs browser. If it's possible to reuse that code it would be a good starting point. Alternatively, you can check the source code for mediawiki to see how they implement displaying page diffs.
Also, if you're interested in using mediawiki format with xowiki or other apps, I have some code you might be interested in. I've separated the parsing and rendering functions that handle mediawiki text so that they can be used in xowiki (or anywhere). I'm still debugging a few things. I should have some code on the web site within a week or so for others to download/review.
