I tried to do this last week, wanting students only to assess themselves, and realized Workshop didn't seem to be about this sort of thing. (Which is fine! I just set up my first Writing Workshop ever before writing this post, and am very pleased with what Workshop promises for peer-learning and rubric internalization1. )
Anyway, it left me thinking afterward that an Assignment, with Advanced Grading > Rubric, and then Permissions override for Student role to Grade Assignment (for just that assignment) might be a close second.
Just a thought. Haven't tried it. Curious if anybody else has?
1 Here's a screenshot of my first Workshop, for posterity. Thank you to the developers.