Allow a note or comment on a previous version of a page

I have a client that often has the desire to add a note to a previous version of a page after it has been changed. It would be good to be able to make a note about a particular old version of a page such as "Do not delete," etc after the edit has been made. Comments merge up to the current release, so offering the user a dialog that they are about to comment on an old version of a page and asking them if they really want to do this or have the comment go on the most recent version would do the trick.