Thinking about secondary saints in the 1962 calendar, today 28 April being a case in point: the General Calendar has St. Paul of the Cross, who happens to have an entire Mass of his own with the exception of the Gospel (which is from St. Titus). I have found the texts of St. Louis' propers in the Missale Romanum 1962 (pp. [150-152], in the Missae pro aliquibus locis) but have not yet located the scores. app.neumz.com has the scores and recordings of the OF chants (which are entirely different).
Since the Missae pro aliquibus locis chants are scattered all around, I have not yet found these propers in what is readily available to me (including many resources downloaded to my computer).
Arthur Connick has worked on compiling the supplements to the Liber Usualis. It's possible that a Mass existed, though it's also possible that the Congregation of Rites simply specified a pre-existing Mass. In cases like this, you could also reach out to a few places: the diocese of Mechlin-Brussels, as Cardinal Mercier was devoted to him, and I'd be shocked if that just disappeared after the cardinal's death. Then there are the congregations founded by de Montfort himself. I'm also pretty sure that he'd be on the local calendar of a few dioceses in France.
But, that said, it doesn't necessarily mean that there are chants, alas.
I do not have notated chants for the "Proprium Sanctorum pro aliquibus locis" Mass for S. Ludovici Mariæ Grignion de Montfort. It is one of thirteen Masses in the 1962 Missale Romanum P.S.P.A.L. section that are still in the wind. The ones I have are on the Institute of Christ the King Sacred Music page here: http://www.institute-christ-king.org/resources/sacred-music#sanctorum
Chants in Gregorian style for the feast of St. Louis de Montfort were recently composed by David Hughes. More information (including scores and performance) in this article at the New Liturgical Movement.
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