I would like to provide feedback via a rubric, but the students will just earn a completion score.

It takes a bit of time to set up initially, but you could use a Selectbox grade item for this.

In the Grades tool, go to Schemes and create a new scheme with the ranges that you want available to you (ie. Complete, Incomplete, or whatever language you want your students to see; minimum of 2 ranges).

Then go to Manage Grades and create a new item that is the Selectbox type. Give it the same name as the assignment, fill in the other fields that are relevant to you and choose the Grade Scheme that you just created.

Now you can link your assignment to this grade item that you created and when you enter the grades for your students, you can fill out the rubric, which they will see, and instead of choosing a point value, you'll just have a dropdown list of the ranges in your selectbox grade item (ie. the ranges from the grade scheme).

I hope that helps!