As schools change their grading policies to eliminate the giving of zeros it makes sense to have an option to "mark all un-scored as missing and incomplete". This would be an additional button leaving the "mark un-scored as 0 and missing" or a way to change the function of that button.
This option would be in response to latest educational practice which says zeros are unduly punitive and don't inspire students to follow up and do the work. An incomplete is no grade so if the student wants a grade they have to do something.
I believe you could have a clone of the "mark un-scored.." button which scans for un-scored but instead of placing a zero it would place an I.