If the assignment does not require students looking at other students work then it's not a "workshop".
What you want may be an Exercise. I've set an Exercise where the students are asked to improve on a given piece of work and submit their "improved" version for grading. The thing to be impoved is a "resource" which they have to download
first. (It's actually a little bit more complicated as I use multiple variants of the Exercise, each variant points to a difference resource file.) The exercise has not completed yet but it appears to be working OK.
If you have many pieces of work they could be set up as resources and the exercise could give the students a free choice on which to download. The students then add their comments to the file and send the chosen one back to you. The assessment form would give the student extra clues as to what sort of things you are asking them to comment on.
In other words, an exercise does not have to start with nothing, it can ask the students to start with something. That "something" may be in the exercise instructions itself or, as in the case of my exercise above, a separate (resource) file.