I just got an excellent ab DVD with three, 20-minute workouts using the stability ball, which are progressively more difficult. It starts with and intermediate/advanced routine, then phase 2 is advanced, and phase 3 even more advanced! I know it's not as short as you are looking for, but each exercise is a chapter point, so you could skip some of them. And there is a 5-minute warm-up with each workout, which you could skip if you do it after another workout (though the stabilization and core activation moves are very nice!
Oh, I guess I should tell you what it is!
"Swiss Ball Abs" by Adam Ford (who is a former professional biker, and works as a neuromuscular therapist--a type of PT? the cues he gives really show his background in PT: tongue on the roof of the mouth, behind the teeth when doing crunches, for example). I got it at amazon.