This idea has been developed and deployed to Canvas
In other Gradebooks, we have been able to enter an "M" is a student hadn't submitted an assignment. It would read the grade as a zero. This is similar the EX feature you already have in Canvas. I know in the individual view I can changed all Ungraded homework to read as a zero but this isn't practical if I don't grade the homework the same day it is submitted. My teachers only enter a zero if student was present and that's grade they earned. If they are absent, etc an M is a great way to distinguish they are still allowed to turn in the assignment but still affects their overall grade if it isn't turned in.
:smileyinfo: We recognize that the specifically requested "M" was not implemented. M, like EX, is not a standard symbol globally, so the feature was intentionally designed without it. EX was already in place and the decision was made not to remove it. We do not plan to add any additional non-standard symbols. EX actually causes a lot of confusion and problems for our international users 😞