I encourage you to read through the OP first on why your suggestion (remove copy-modification entirely) is not reasonable.
> The metro.co.uk code inserts an off-screen-positioned element with the disclaimer into the selection range, and you could just listen for selectionchange and/or mouse drag and/or keypress events to detect selection changes, and add/remove such an element based on those events.
Well, shit.
Never underestimate newspaper publishers' CMS vendors' ingenuity.