You can write as many test cases as you'd like using JAWS, even 508 of them. All kidding aside, it's important to keep in mind the end goal for 508 testing. Of all the testing types that we do, I think 508 and usability really is one that is often overlooked and has a significant impact.
Can you write them using JAWS? Yes. You could. The question is should you. Best practice would be manual testing here, to emulate the user experience as closely as possible. If there is a usability frustration or stumbling block, an automated test solution could easily overlook it completely. Your product would be deployed, but the customers that would have been impacted -- will be impacted.