@James
We don't believe so, since we are trying to take exactly the value of the attendance grade off the top of the current final grade.
That was how we initially attempted to solve the problem, using the 'update_grades' Submission endpoint. We created an 'Attendance adjustment' assignment group with the associated assignment and set the group weight appropriately.
However, we ran into the issue of how assignment group weights are adjusted when an assignment group has no grades entered. Since it removes the assignment groups without grades in them from the final grade calculation, the weight of the assignment groups that do have grades in them get modified. We experimented with creating place holder assignments in every assignment group, so the weights remained unchanged throughout the course. But once real grades were populated in an assignment group, you now had this 'dummy' grade being averaged in.
I think the solution you presented is as close as will get until other features are added. Thanks again!