I assume those students are not signed up for the AP exam then, right? If they are, I'm pretty sure not submitting either PT counts as not taking the exam (so regardless of how well they do on the multiple choice, it counts as they didn't take the exam at all).
I personally would grade their tasks according to the rubric included with the Create PT. You can collect their work using the College Board Digital Portfolio, just like how all other students will be submitting them. The students don't need to be taking the AP exam to create an account and upload their work.