On a question with votes to close or a question or answer with votes to delete, the diamond moderators should be able to see who these users are. This information becomes public record if the post changes state, so there's no reason to hide it from diamond moderators. This is particularly useful to identify users who are (or have a history of) closing valid questions on a site.
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I strongly question the whole idea of 'abusive' close votes. The reason these things are 'votes' is that people's opinions may differ. We tell people, over and over, that their votes are their own, to cast as they see fit. If Zorgon from Jupiter is sending them radio signals telling them how to vote, that's OK. I think that close votes should be viewed the same way as post quality votes.
For close votes to become abusive, then, I think that it requires sock-puppetry or conspiracy.
If mods want to ask for tools to help detect 'closing rings' or some such thing, I have no objection.
As for delete votes, I'm more conflicted; I think that we try much harder to achieve some consensus on what gets deleted and what does not, and orders from Jupiter are not so good.
While I don't think that information is particularly private, I disagree that this is necessary or that your motivation makes for a good argument for implementing this.
You can identify potentially problematic closers by looking at the closed questions, not by policing close votes that might not even lead to a closure.
Further, account annotations and mod messages shouldn't be your first steps here.
If you see good questions that are getting close votes, leave helpful and educational comments that explain why they should remain open instead of trying to hunt the specific users. If you see a pattern of questions on particular topics getting closed, open a meta discussion and link to it from the comments.
That way everyone benefits from the experience and can learn to either reopen wrongly closed good questions or vote to "leave open" from /review and help prevent closures in the first place.
I'm not really opposed to letting moderators see that information. Like you said its not really secret anyway. The problem I have with this idea is that I can't envision a way that this extra data would be actionable by a moderator. A close vote is only 1/5 of actually closing the question so even if a user is voting to close good questions others have to agree with them for anything potentially negative to happen. When that question becomes closed it's not not necessarily a bad thing as it was the community's will being done and the community still has the power to change its mind. If a question is closed by the community it is by definition not acceptable in its current form, if there is in fact a group of users attempting to subvert the will of the community there would be evidence of the community fighting back by re-opening (or attempting to) good questions which would be the data you should be taking action on.
I don't really want to get into this debate, but part of the problem with a community driven site is that the community (or a large enough portion) will disagree with "the powers that be" on what is acceptable, for example if tomorrow SO decide bedazzling your mouse is on topic, then problems are going to occur. As a moderator you aren't supposed to be an arbiter of what is acceptable on a site, but you are given some power to act as one because reality dictates that necessity, and asking to be able to review this information really seems like an attempt to become a judge of what is correct behavior with respect to closing.