To clarify the proposal is:
1) only looking at new edits that add blocks of text over a certain size
2) only tagging those edits on a workspace page for further follow-up by an experienced human editor
3) only running on articles of WikiProjects that want it and are willing to follow-up (thus only WPMED for starters)
What it is NOT is: a tool to add notices to article space, a tool to warn users on their talk pages, or a tool to look at old edits. It is also NOT many other things. This is a very narrow proposal.
With respect to users who are adding content they own which they have previously had published. What you do is you get them in an email to agree to release it under a CC BY SA license and then send that email to OTRS.
With respect to the number of edits, WPMED gets about 1000 a day. If we say about 10% are of a significant size (a rather high estimate) and if we say copy and paste issues occur in 10% with a same number of false positives we are looking at 20 edits to review a day. Those within the project are able to handle this volume in a timely manner.