Student-level grading scheme
Thanks for posting this in the global Canvas Community!
I looked into this for a little bit for you. What I found was that you could set up a Grading Scheme for the specific assignment. Essentially, you would select Letter grade as the Display Grade As area of the assignment in edit. From there you can see an option to View Grading Schemes. You would then assign the assignment to the specific students. This should make it so that only the students assigned to the assignment will be graded based on the scheme you select. Of course. you will need to setup a duplicate assignment for the students you are not using the scheme to grade with. This might not be the best option if you are using group assignments and discussions, unless the students you are using the specified scheme with are all in the same group.
HERE is a guide on how that works for assignments.
HERE is a guide on using grading schemes in a course.
Other than that, the only other option I could think of would be to move the students to a new course and set the grading scheme up in that course to be used. I was able to locate THIS presented idea that sorta touches on allowing Multiple Grading Schemes to be used for each student.
Hopefully this helps!