The term I was looking for is "singmesse". And apparently there are places in Europe (and America! https://www.oldstmarys.org/about/sacred-music-at-old-st-marys/) where it was never stamped out.

"low-Mass-with-hymns" is common in Switzerland they have Hymn books published c.1900 and a new version published within the last 20 years. These have suggestions of German songs that can replace the Ordinary and Proper.

Is there any evidence that such a "low-Mass-with-hymns" format might have survived in ethnic parishes after Vatican II?

Here in Czech Republic the "pastoral masses" for Christmas (the one by Ryba being the most popular, but by far not the only and probably not the highest quality one) are tolerated, despite violating all liturgical norms, because of their immense popularity and being a treasured part of the national musical patrimony.

The much simpler "Singmesse" repertory of hymns sung by the congregation with organ accompaniment, was after Vatican II rearranged and is still used as the most common "alius cantus aptus", replacing chants of the Mass proper (never more the ordinary).

I'm trying to dig into a mystery: a group of Czech hymns written by an obscure Czech-American, which have Ordinary titles above each, but which are dated 1980.

An old man composing a Christmas Mass in the vein of what he was used to in his homeland back in the days of his childhood, cca 1920'-40' (you mean the Christmas Mass by Jan Burian, right?), ignoring later development of the liturgical norms. (Or possibly relying on the fact that their violations would be generally tolerated when performing pastoral masses like the one by Ryba.)

In Italy we had something similar, there was a collection named "Il Parrocchiano cantore" (the parish singer, roughly) created by Monsignor Rostagno and the priest Giovanni Pagella, both from the Salesians of don Bosco, which included gregorian chant in modern notation and some hymns for introit, offertory, sanctus, [before] the elevation, communion, and after the mass in venacular