@MJO
It is a part completed project of mine to have an index of melodies of Chant Hymns arranged according to metre... Sadly this is one of those projects that suffers from mission creep. While it is a small thing to set all the melodies found in the Liber, but then there are additional melodies in the Antiphonal, the Matins melodies?

Q. What about the revised melodies in the Monastic Antiphonal 1934?
Q. Do we add the even more revised melodies in the Liber Hymnarius?
Q. While the Monastic Antiphonal, and the Liber Hymnarius were the latest research, others have gone further...
Q. Do we add the pre-1911 melodies? Dominican? Franciscan? Sarum? older melodies from the Cantus Database?

Then what do we do about the chant melodies that are variations on an ancient source where the Monastic, Roman and Dominican melodies have differences?

As for the 'Chant, Mode V' question, it is common to attribute the text and music in Hymnals. A quick look at the Analecta Hymnica or Connelly / Britt will give you a date and author for the text, the melody is so much more difficult. We usually don't know who wrote the melody and we don't know the date as some melodies were composed pre musical notation!

The closest we have to a database of melodies is this, http://www.globalchant.org/search.php sadly it is not a complete index of the melodies...

@chonak I suppose it's evidence of hymnal editors who have only a vague knowledge of chant and don't know where to look for more information to give. You're right that in these instances they should at least say what book they copied it from.

A demonstration of a hymnal which does much better: The New English Hymnal. For example, see this instance of a hymn with plainsong tune. The tune is assigned a name like any other hymn tune. In addition to these tunes appearing in the alphabetical index of tunes, there is also a plainsong metrical index, where it can also be seen that one tune is sometimes used for several texts.

So perhaps assigning names to tunes is the way to go. Certainly some tunes have names already from the texts they are inextricably linked with—Veni Creator, Vexilla Regis. I seem to remember that variations on tunes are sometimes subtitled with their origin—Roman, Monastic, etc.—but variations aren't such an issue. Modern hymn tunes often exist in several variations, as was recently demonstrated in the forum harmonisation challenge. Perhaps the origin of a specific variation should just be added in parentheses. But how one would assign a name for a tune used for manifold texts I don't know.

@tomjaw, your project does sound very useful. It sounds like the kind of thing which could benefit from crowd-sourcing. Perhaps it's a feature which could be integrated into gregobase.

... is knowing the mode supposed to be helpful to readers who want to learn more about a piece? I think this is not very useful: as if the melody info for a hymn were "Chorale, D major".

I agree; that's not helpful for knowing more about the piece, but it is helpful for performing it, as are other notations like "Moderately slow" and "CMD". When the editors say "Mode 5", I think they're assuming that many readers will waste time foguring out the mode for themselves, or that the readers might not be able to. If they felt that many users would have trouble figuring out a piece was in D Major, they would spell that out (as some shape-note collections, designed for singing schools, do).

There are Tone 1-8 and Mode 1-8. Brager's Chant Motet Book uses both. I associate Tones with the psalms. In the same book Attende Domine and O Sacrum Convivium are set to Mode 5, however, note for note they are not the same. Specifying Mode 1 or Tone 8 doesn't tell you anything about the origin or source.

What if we gave ever paiinchant hymn tune a name. Some are enough well known that they are recognised as Veni Creator, Adoro te, Te lucis ante terminum, etc. Most, such as Verbum supernum, or even Conditor alme siderum, would not be known by most - which is no reason not to begin using them. After all, relatively few of the modern tune names are on the tips of most people's tongues. For how many does This Endris nyght ring a bell , or even St Albinus, etc. One will find that in The English Hymnal the plainchant tunes are, indeed, labelled by their incipits - in addition to their mode and meter, both of which are useful information. The mode is more useful in identifying tunes in plainchant than a key is in identifying a modern tune because so many of the modern tunes appear in multiple keys (though mode, as in major or minor, might be useful in tracking a tune down - and consistent with the mode labeling of plainchant tunes).