I like the Preludio pentatonica very much too. I haven't worked at it yet as I was focused on the tasks at hand, but I did play through it a couple of times. Feel free to post a recording of your Preludio. All the pieces played at the Lesson intro are fair play, but only the listed ones count for the requirement.
About the Ostinato, have you tried playing it with a capo to try to find a fingerboard position that is within reach for your fingers, say 5th fret or so?
If music be the food of love, play on. --Shakespeare, Twelfth Night, 1.1