Adding buttons to built in property inspectors is problematic, even if it were easier, it wouldn't be the a great way to handle it as the tooltips are applied throughout the page, not to individual form fields. As the property inspector only appears when you have an individual Spry field selected, it would give the impression that the tooltips were being edited for that particular field, not all of the other fields which is what would really be happening. As Dreamweaver doesn't provide for any neat and tidy way to inspect such applications, we felt that accessing the Insert bar button again was the best all around solution. Not perfect by any means but better than the other options available.
Thanks for the feedback.