It seems that this issue was put on ice but I have made a plea to revive it because it has caused a lot of lost time for me. If this is something you recognize, please vote for the tracker. The following is my response on MDL-25620:
As described in http://moodle.org/mod/forum/discuss.php?d=194659#p848092, the issue is more about user friendliness than capability.
Yes it is possible to set a whole category in the gradebook to aggregate including empty grades. But this causes unwanted consequences.
1. What if that same category contains several offline assignments or manually graded assignments? Students' grades would be very low until the teacher graded it. If the weighting for an ungraded assignment was set to zero until it was graded it might work.
2. However, this means that teachers would have to change the weighting every time a new assignment was given to the students. After recycling a Moodle course I would then have to return the 200-300 assignments to zero weighting...not fun.
3. The issue is further exacerbated if you have different groups with different assignments. We can't control which group and which assignment will/will not count using weighting and aggregation including non-empty grades.
Additionally, with younger students a blaring zero is needed to emphasis that that particular assignment must be done or was missed.
I envision a solution similar to the 'upload a file' assignment with the following options for teachers:
1. If student does not do assignment by due date - give zero
2. Allow for possibility to do assignment after due date with penalty and option to email notification to teacher.
3. To differentiate between a real 0 and a non-attempted 0, student can get feedback - NOT COMPLETED. Perhaps this can even be a user input field like with the grade boundary feedback.
4. Select which groups these conditions will apply to (including the ALL GROUPS option).
Bottom line, as teachers with 100s of students, we rely heavily on the automated feedback that quizzes can give. However, if Moodle doesn't clearly tell these students that they didn't do the assignment, the teacher, once again has to do the chasing.
Please reconsider creating a fix to this problem. It seems to be one of the few issues with Moodle2 that stops me from using quizzes on a daily basis.