We have this capability with Moodle because we use the LAE Grader report and it has a 'zero fill' icon at the top of every grade item column that fills all empty grade cells in that column with 0. If you need it autofilled with something other than zero you simply need to put that value in the offset on Categories and items.
New gradebook interface
It would be amazing if LAE grader would just put an entry box at the top of each column and allow you to put a grade in there to autofill. However, I don't think this should be linked to the offset column because that would somewhat defeat the point- I think it should just fill the grade field as a regular entry. The value of the autofill column is that there are a lot of assignments where the whole class gets the same grade except for one or two students. Then you could autofill with the majority grade and just change the odd ones.
I am having trouble making the offset work. I have an assignment worth three points, and I want to fill all the fields down with 3. So, I put a 3 in the offset field in the categories for that assignment, but it still filled the fields with zeros.
What did I do wrong?