Privately sharing an unused peer-review report?

In our most recent "how can we help you?" thread, Erik Magnussen writes: 

I was recently asked to review a paper and was halfway through doing so when I realized that the author had revealed their identity through a footnote. I quickly contacted the editor to let them know that anonymity had been breached, was thanked for bringing it to their attention, and was subsequently let go as a reviewer (as I think I should have been).

My question is this: would it ever be appropriate to forward my comments to the author in this type of case, or would this be an inappropriate interference in the editorial process? I thought the paper made a valuable contribution to the debate that it centered on, and I had a number of ideas about how the author's argument could be improved, which I had started to outline in my report. These may still be of use to the author, and I'm wondering if there is any definitive reason to keep them to myself.

To be clear, I have not forwarded anything to the author, and don't really have plans to. But it made me wonder about the ethics of doing so, and I know that the community here will have some thoughts on this. Thanks!

My initial reaction here was that this seems like a straightforward case: once a reviewer assignment is over, you're sort of free to do whatever you like (provided you otherwise stay within the bounds of general professionalism). However, after thinking about it for about 5 more seconds, it occurred to that the prudent thing here is probably to ask the Editor in Chief of the journal that commissioned the review to see if they have any objection. This occurred to me in part because, in my experience, some journals have an explicit policy that the peer-review process is supposed to remain confidential, and you never how exactly how this requirement may be understood. Sure, it may be unlikely that the author you reach out to you tattles on you for it (particularly if, as in this case, you let them know that you liked the paper)--but, I guess in general, my general maxim on professional uncertainty is this: "If you are in any doubt, ask before you act!".

But these are just my thoughts. What are yours?

Comments

[Anca Gheaus]

I don't see why you shouldn't do that. The author, and their readers, would have reasons to be grateful for this gesture.

[author author]

I have a different intuition on this. Clearly, the author should have kept anonymous. But given they did not, the referee should have either (i) just completed the review, or (ii) step down, as they did, BUT not contact the reviewer. The point of them stepping down was that they felt they should not review the paper - well it is a similar breach of confidentiality to now contact they author. They are not supposed to know who it is.
Incidentally I publish in a "scientific" field as well. When I submit manuscripts to journals we are explicitly told to provide full author information on the manuscript that is sent out for review.

[Erik Magnusson]

@author author:

One might argue that the purpose of anonymity in the editorial process is to ensure the integrity of the review process and protect the author from a biased review, not necessarily to protect the author's privacy. So while it would be inappropriate to be in a position to determine the fate of a paper with knowledge of the author's identity, it seems like an open question whether it is inappropriate to make contact with the author after one has stepped down. It might be, but not for the same reasons that justify stepping down as a reviewer (or so it seems to me).

[author author]

Erik
The only reason you had access to the author's paper is because you were asked to referee it by a journal. So contacting the author is inappropriate, given that she has only committed to communication regarding the paper with the editor. That is how I see it.
As I noted, I publish in another field where the norm is to disclose the author's name. In that context, there is no pretension of author anonymity.

[RJ Leland]

I think it’s fine (awesome even) to share.