Three thoughts initially come to mind:
1- communicate about the
assignment through the "Submission Comments" instead of the 'Feedback
Comments'. This can be found on the quick grading page if
turned on in the assignment's settings. You'll have to click 'save comment' beneath the text you enter since rubrics don't have a 'save all quickgrading changes' option.
2- If all your students will be submitting around the same time, then turn the rubric off for your assignment for the first phase, and just leave feedback without a grade. Then when everyone is ready to submit the final version, turn the rubric back on and go in and grade using the rubric.
3- use 2 separate assignments (the first
upload uses simple direct grading where you can leave feedback and no grade, then the second upload uses the rubric)
Also, I've never used the Workshop Module, but maybe that is what this module is intended for.