Put another way, on a website like draw.io, you need to be able to hook into copy to allow lassoing and copying a set of shapes (and have them remain editable when pasted).

The entire ask (stopping the copyright, and allow clipboard integration) be achieved by allowing pages to augment the clipboard data, which can already be done, but not replace it. Pages would still need to be able to hook paste.

Being able to hook paste is a much more reasonable ask. If you have js enabled, they're going to be able to access the content after paste anyway, so it's reasonable to allow notification that a paste has occurred.