That would require writing extraneous info to the files, and then expecting everyone to remove them afterward, just to provide a halfway approach to
showing talking about a change diff. And then you have to go around searching for the comments.
A better idea would be to use actual diffing.
calibre can do it. In fact, calibre's editor can also compare any two books that were edited by other tools.
Perhaps someone will formulate a diff feature for Sigil, though I wouldn't hold my breath.