well, that is incredible hard. This just counts number of words in a specific revision. I'll see if I can add a diff between the previous edit and this edit.
I'm looking here at just PostHistory with an event 5 (that is body edited). The text field will have the complete text for that new revision. I got the number of words by splitting the text on a single space. What I can do is find the previous revision, spilt on space to get that word count and then subtract the two. That is about it what can work for a single user.