I have been working recently in some late-eighteenth century a capella motets for Holy Week. Not unique in Portugal, the composition of this kind of works seems to have been a common practice in Portuguese cathedrals and other churches with an established musical activity.
Although for the Portuguese case I do not know of any significant studies on this repertoire, the Brazilian case has seen quite a lot of editions and studies mostly because almost all significant Brazilian composers of the so-called “colonial period” have written works.
That is the case of José Maurício Nunes Garcia (1767-1830), author of several of these motets, for four voices a capella. I found several live recordings by the Brazilian group Quarteto Colonial of great vocal quality. This group toured Portugal a couple of years ago with an interesting programme featuring these motets by Garcia.