in mod/assign/locallib.php
in the method get_user_grades_for_gradebook()
the function can be fed a $userid of 0 for all users. This SQL will then select all users in the entire system instead of all users associated with just this course assignment.

 SELECT u.id as userid

	, s.timemodified as datesubmitted

	, g.grade as rawgrade

	, g.timemodified as dategraded

	, g.grader as usermodified

FROM mdl_user u

LEFT JOIN mdl_assign_submission s 

	ON u.id = s.userid and s.assignment = :assignid

LEFT JOIN mdl_assign_grades g 

	ON u.id = g.userid and g.assignment = :assignid

WHERE u.id != 0

As an improvement I get a list of enrolled users in the course and append "AND u.id IN (#,#,#,#,etc...)" after "u.id != 0".

However I think this could be pointless because of the fix from http://tracker.moodle.org/browse/MDL-36061. Once MDL-36061 is merged I don't see how someone could get to get_user_grades_for_gradebook() intentionally with a $userid=0. Which I think would make this an improvement for something that no longer happens.

Any thoughts?