As I have mentioned before, I was lucky to have in my choir loft a copy of the "Pius X hymnal organ accompaniment" book. I have often used it's chant accompaniments for improvisations, and a few of the easier (2 part) latin motets.
I have not been able to find anywhere online, either then or since, another copy of this, or heard of anyone else who has one.
I also have in my choir loft a choir book of the "St. Gregory Hymnal." A week or so ago, I decided to do a search for the organ accompaniment edition online, to see if anyone has reprinted it (the choir edition is not very useful without,) and lo and behold it has been available through the Newmann Press since 2000! So, I purchased a copy, and it came in the mail today and I am quite excited to skim through it, examining the organ parts, and possibilities of new hymns or things for the choir to sing!