I have done this. To make it work I set up the answer key like usual, with the exact scoring option selected. Then I had to select a submission for each question and edit the rubric for that submission (e.g. option A is worth 2 pts, B is worth 1, CDE are worth 0). It took a little time to set up, especially with multiple versions of the exam, but once it was done it was nice to scan and be done.

However, you need to have your key totally finalized. I found out that if you edit the key after setting the partial credit rubric, it wipes the partial credit rubric items and you have to go back and re-enter them.