In the meantime, I have had an interesting conversation with Dr Beale (@Dr_Teacake) on Twitter. He commented: A brief trill might be right. We lengthen the previous note, *and* often the subsequent notes. When I asked how he decided, he referred me to the Graduale Triplex. I also asked for examples of people employing a trill, and he directed me to two fabulous and fascinating recordings on Youtube, here and here.
Yet again, blogging my ignorance has led the wise to teach me. I love this!