University musicology professors have confessed to ignorance of this -
University organ PhDs have likewise not known
Several liturgical scholars have no answer

So...
I will 'throw this out here' and see if, perchance, someone has an anxwer -
An answer, that is, to the following -

I am planning a recital at Walsingham to take place this autumn on the Solemnity of Christ the King.
One of the items on the program is the Kyrie degli Apostoli from Frescobaldi's Fiori Musicali, which just happens to be the Kyrie Cunctipotens genitor Deus.
It will be performed with alternatim chant.
The question is, would this chant have been troped during Frescobaldi's tenure as organist at St Peter's Basilica in Rome?
The question arises because I am unsure how universally the kyrie was troped at this point in history.
In France it generally was.
It also was at Sarum.
In England troping was so ordinary that kyrie was considered part of the proper of the mass.
Hence kyrie does not appear in many English polyphonic masses of the late mediaeval and renaissance eras.
But, what was the custom in Italy, particularly Frescobaldi's St Peter's?

(The other alternatim item on this recital will be Guilain's Magnificat du Second Ton.)