When GUI Style is set to GTK+, a tristate QCheckBox in the Qt::PartiallyChecked state is indistinguishable from one in the Qt::Unchecked state - unless you set up for non-standard display e.g. by providing special ":indeterminate" style data.

Gtk does support the concept of an "intermediate" state, however, and it should be possible to get a special indicator via its drawing routines - see http://developer.gnome.org/gtk3/stable/GtkToggleButton.html#gtk-toggle-button-set-inconsistent. Adding support for it is probably just a matter of inserting

if (option->state & State_NoChange)
state = GTK_STATE_INCONSISTENT;

at the appropriate spot.