(Oops, a post slipped in while I was drafting this reply to Cdalitz)
I'd like to better understand which aspect(s) you are referring to.
Can you say more about which aspect of the note-for-note setting by the later editor is plagiarized from a creative contribution by the earlier editor, but which is not part of the original public-domain work or one of several likely ficta/underlay/voicing/octivation solutions that people would independently come up with?
Or are you saying that it's not that you know of a creative element in that regard but, rather, that if an editor does the hard work of mechanical transcription (say from old clefs into modern ones), that mechanical transcription is itself copyrighted even if no creative elements are added, or at least should have moral protection if not copyrightable?
I presume you are not focusing on any editorial decisions in the layout of a piece, which are generally mooted when a piece is typeset differently by a later editor (esp. if the sofware makes most of the decisions).
I'd appreciate your clarifying what you are referring to.
I personally agree that CPDL should strongly encourage editors to post about the sources used for their editions. Regarding *requiring* this, I have not thoroughly analyzed the issue of what impact requiring that would have, and thus do not yet have any firm personal opinion regarding this aspect. But why should that stop me?

Here's my own thinking so far:
My feeling is that we collectively don't have enough data about either the impact of such a type of user violation. I am guessing it is rare. Also, impact is further reduced because editors of rare or otherwise distinct editions do have an easy method of making a DMCA claim on if they care to run a periodic search on the specific work). OTOH, we also don't have data about the inhibiting impact on the many editors uploading legitimate scores. I think it would be hard to get real data about this. The latter risk (inhibiting legitimate contributions) is a really big one for CPDL to treat speculatively, especially when the earlier risk (violating the rights of earlier editors whose works might be copied) is mitigated by the ease of searching CPDL directly or having a recurring Google search pick things up once Google indexes a new work and then filing a DMCA claim with CPDL to request removal.
You've always had thoughtful contributions to make and I really would like to hear more about your thoughts on these subjects.