I have a similar problem. A colleague has a student assistant who he does not want shown in the list of participants. Hidden assignment works fine to accomplish that, however when that student assistant (who has the role of non-editing teacher) marks an assignment her name shows up prominently when members of the class view the marked assignment! That seems to defeat the purpose of hidden assignment.
I suggested that one way to get around this is to create a new account for the assistant that would only be used for marking purposes. This should work, but it definitely is not a very elegant way to do things.
Roles and permissions
Hidde Roles in modules/blocks
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