Update: Ginzburg and Sag (2001) have a “WH-amalgamation constraint” similar to SLASH-amalgamation, stating that a word’s WH is the union of its ARG-ST element’s WHs. (WH there corresponds to QUE). So a noun will have its specifier’s WH, for example. Then for verbs, they say that “subject-selecting verbs must be WH-empty”, but that is because they have something called WH-Subject Prohibition: a word’s SUBJ must be WH-empty. This is so that sentences like Who left are licensed by the HFR and not the HSR. They have yet an additional WH-Constraint stating that “any non-initial element of a lexeme’s ARG-ST must be WH-empty”. It is a bit confusing to me; the section on verbs is disappointingly short and focuses entirely on special cases, not really explicating how the basic cases are supposed to work. Maybe I will find that in an earlier section (though I already have read them!)