I cannot help you with this, but am curious about the Spanish Californian examples you mention. Can you put examples up here.

So far as I know 'psalm tone propers' are a development of the early XXth century. No doubt there are some here who can speak more authentically on this subject.

I have been told (so this is simply third hand evidence) that at the missions, since they had no books, they would simply sing the Introit for the Feast of All Saints "Gaudeamus omnes in Domino diem festum celebrantes sub honore ______________" and then fill in the genitive form of whoever the Feast belonged to. In the present Gradual I believe
that these examples are still included: "Mariae Virginis", "omnium sanctorum", "beatae Annae" , "Thomae martyris". How they handled the other propers, I have no idea.

gregp, that is not correct. They certainly had music books at the Missions! Most did not survive; still, the surviving corpus constitutes the largest collection of colonial Spanish music north of Mexico. Gaudeamus was the model, but, as you will see if someone can show me how to upload a picture, the music was written out.

Interesting, indeed!
These have a rather 'late' air about them. I would like to know what others more knowledgeable about colonial and provincial chant musicianship would have to say about them. What jumps out at me is not so much the multi-textual use of a tune (which we carry on about quite a lot with regard to post-Gregorian, harmonised hymnody!) but other evidence of notational degeneracy. To wit, the absence of podati, clivi, and other such neumes, and their replacement by just right to left note after note notation, plus long notes to take the place of repercussive ones, plus a mis-use of the rhombus to indicate (presumably) a quick note (speech rhythm?). All this is evidence of a degenerate chanting style - degenerate in relation to presumed very early, seminal, methods which we associate, with period variations, with the last thousand (at least) years. While the five line staff appeared quite earlier than many think, as early as the XIIth century, these examples are obviously not older than a very few hundred years, if that.

Most here will certainly know that this Gaudeamus chant appears with multiple texts in the GR. However, it is indeed curious that it became in mission territories an, as it were, 'default' tune for a great number of texts. Rather modern a practice, wasn't it? As if they were saying amongst themselves - 'oh, we'll sing it to this tune that everybody knows' - some things never change, and I'm sure that some of us, had we been there, would have wretched at the practice.

I probably haven't helped you much, Steve, but what you have shared here is quite interesting. Perhaps others will do it more justice.

And this is just one choirbook, but in general, one will only find the breve, long, and rhombus (along with the dot) in Durán's choirbooks. In choirbooks by other scribes, the virga and various joined neums do appear, but the notation is not more varied. I see a nacent rhythmicization, and there is indeed at least one work, a Credo, for which we have a free rhythm chant-like setting which later appears on both sides of the Atlantic in a fully-rhythmic version, canto figurado version.

In the mission repertoire, the use of the rhombus, bread, semibreve, and long come from the prontuarios of late-Renaissance and early-Baroque Spain (which were reprinted through at least the end of the nineteenth century). In the chant repertoire of the CA Missions, the rhombus is used without regard to the direction of the melody.

The organ you mention is a fabulous instrument, in the Mexican-Ibrerian tradition. It was Fr. Durán who initially ordered the organ, on January 7, 1821 (Manuel's site lists the wrong date.)

Also, the choir book from which the above photo was taken is not the Bancroft choirbook, but a different Durán choirbook. The identity of the original composer or composers are not known; Durán was certainly not one of them. It is more likely that many successive Friars have had a hand in the shaping of the repertoire as it has come down to us.