I know someone who is VERY interested in this hypothetical antiphonal. He wonders if it has already been assembled and, if so, if it might be available privately--for only the most non-commercial and hush-hush of viewings, with monitor brightness turned down low and curtains drawn. What should I tell him?

@Olivier I know someone who has been very slowly assembling it - right now only beginning with the psalter - for his private use.
If he eventually decides to commit the crime(?), he will publish the resulting files as well as sources. (With hope that someone might eventually join him in the effort.)

@Salieri Yes, St. Gregory and others are. I was not totally sure to which extent Psalterium monasticum contains neo-gregorian compositions. But the preface stating that "Omnes huius libri melodiae ex traditione gregoriana proveniunt ... Antiphonis, in Thesauro (sc. Thesauro liturgiae horarum monasticae, 1977) quidem propositis, sed ab illa traditione alienis, aliae, similes tamen quoad sensum, substitutae sunt."

I should update my website and state that the booklets for the Liturgy of the Hours are outdated... and that I have no time to update them...

For one of my hardcopy booklets of Compline, I asked and got an attestation of my local ordinary. I think that for printed books that are to be used in the liturgy you should always ask this permission. Same holds for a compilation of an Antiphonale according to the 1983 OCO (but why would you create this in the first place, instead of one according to the 2015 OCO?).

@smvanroode "... why ... instead of one according to the 2015 OCO?"
I hope (too naive?) that in a few years I will hold in my hands (or at least say "if I had the money, I would hold ...") a beautiful new official edition of the full Antiphonale Romanum. And I don't ... er ... THAT person I know of doesn't ... want to create a concurrent edition.

Also, I find the fact quite scandalous, that an antiphonale according to the OCO 1983 seemingly hasn't ever been assembled

The problem for publishers is why should they compile and print this...
1. How many times has the Office suffered major changes over the last 100 years?
2. How many versions of the Office are in use, how popular would a Latin version be.

This post and comments is most informative,
http://liturgicalnotes.blogspot.co.uk/2016/01/pope-francis-on-judaism-and-liturgy-of.html

Regarding the Graduale 1974, I don't find that it has a concordat cum originali; rather it only has an imprimatur. Ditto for the Liber Usualis.

On the other hand, the Graduale 1961 does have a decree testifying to its agreement with the original sources.

But as mentioned, some books by the CMAA do not even have an imprimatur. (e.g. PBC 1st edition)

It makes me wonder: What is a book with a concordat useable for, that one with merely an imprimatur would not also be useable for? (technically / legally speaking)

Or one with an imprimatur versus one without?

Reproductions of the editio vaticana chants and reproductions of chants from manuscripts... these might have particular issues I'm unfamiliar with. Over the past century the law has changed a bit, as the old Index is gone and the Code of Canon Law was reformed in the 80s. But the current practice of ecclesiastical censoring, as I understand it (after doing some work on this for a religious institute's press) is as follows:

An Imprimatur is the typical (but not only) way to show that the local bishop has permitted/approved a book to be published (usually via an agent and a censor).

Current (canon) law gives the local bishop discretion for publication of "new" editions of liturgical books (Can. 826 §2), other prayer books (Can. 826 §3), new translations of the Bible (Can. 825), any other books that deal with faith and morals (Can. 823), and some others. (Obviously these canons don't/can't bind non-Catholics and secular presses.)

A Concordat cum originali is a particular type of permission/approbation that is used nowadays for each particular edition of liturgical books that reproduce a previously approved text: "This isn't the editio typica, but the words are the same." Most liturgical chant book collections, that aren't strictly scholarly or historical, seem like they would fall under this category.

It doesn't so much limit the use of a book though, as it limits the publication. A bishop's permission doesn't guarantee to the user either the accuracy or moral content of a book, unfortunately, and most modern books even print this fact along with the permission. I suppose for a liturgical book there is still a moral responsibility of the user to be reasonably sure the book is reliable and accurate before it is used publicly.

But if you are morally certain that your own hand-copied missal is accurate (or make use of self-printed readings from the USCCB website), I don't see what stops you from saying Mass from it, save if it might reasonably scandalize people. But to publish it for the use of others... the law just needs you to get the OK of your bishop, because before the law and God, he's the one ultimately responsible for the liturgy in his diocese.

Since the Holy See never published an official Graduale Romanum after V2, there was no "original" edition for the GR 1974 to match. Thus a concordat cum originali would not be applicable.

Instead, the Holy See published Ordo Cantus Missae, which provides the schedule of which chants belong on which observances. So the GR 1974 was constructed according to that schedule, drawing most of the chants from the GR 1961, and with Solesmes making a few modifications (e.g., dropping some late "neo-gregorian" compositions from the book; see the footnote on p. 8 of GR74.)

There is already such a database, which I think could host these chants: http://gregobase.selapa.net/scores.php.

@awilliams For the "1983 antiphonale" I use a github repository as a "database". Contributions are possible and welcome :)

Critical comments that would best be sent as a private message instead of to the list do more than just clutter things up here, they also dramatically result in others never posting any music that they have created for fear of being raked over the coals.

True, there are some people who know more than others do, but sending such comments as a private message makes clear your intent to help. Posting it in public may not.

As the angel who lives on, free to visit here since the blackboard no longers needs to be cleaned on Fulton Sheen's show, above the angel clearly said:

Here's a routine reminder: Be discriminating but don't nitpick.