We understand and thanks for your quick response. The teacher in my school who wants to hide grades from students has this purpose. He wants to make grades as notes to himself for later determining a final grade. He wants only written feedback to go to students so they are motivated not by a score but by a desire to explore and dialogue.
As a workaround, he is planning to initially set "no grade" as you suggested. Then when the activity period is over, he will hide the activity and do his grading. This is requires more time by going through the assessment twice, so we hope you can consider a complete hiding in future versions.