Thank you, I understand how to set a playing technique, but how does one set it for a specific note length when there is nothing shown in the default dialog? Where are the open vs. closed noteheads coming from?
Well well, mea culpa! At some point in the past I added an open notehead for anything of 1/16 (semiquaver) in value. I found it in the notehead set editor (Library > Notehead Sets).