As you know, func_detail is an "internal entity", meaning it just tells VBSP to ignore those brushes when calculating vis and otherwise treats it like regular world brushes. It doesn't count towards the entity count, nor go invisible when you clip outside the map, the way func_brush does. Unfortunately, unlike displacements, func_details have no option to be non-solid. Would it be possible to add that ability with an FGD tweak? Either by modifying the entity's existing attributes or by creating a new internal entity with otherwise identical properties.