Hello Daniel,
I don't know the official answer, but I would first try to use the eight note as pulse unit (la) so that two sixteenth note become lir-ra (and the fourth note: long).
If the sixteenth notes occur rarely, I personally (when studying a melody for myself) sing them as a neum ("Li-i" or "ra-a").
Another answer is that if you come to sixteenth note, you are sufficiently proficient in music not to use a metric language.