Depends on what is affected. If you have properties only affecting
noteheads, the note-head-interface is the place for them.
Ah, okay. The one we were talking about would be needed both on notehead and
on stem grobs, and there may be another one with the same distribution
(although that might end up being defined only inside the "mensural.ly"
stylesheet, where it wouldn't have to be tested for belonging to an
interface at all – unless you tell me that would be a hack to be avoided).
By the way, again about naming the property in question, would
"hollow-duration-min" be okay?