Why are you basting it? What are you basting it to? If you are looking for a way to keep each scallop down as you quilt that section, I suggest you use spray adhesive AFTER you've rolled to that point. There is also a wash away doublestick 1/4" tape (I think Joann sells it in notions). You'd have to explain that to the customer so she wouldn't try to pull it out. Otherwise, you could pin the scallops in the current section and remove the pin from the scallop you're quilting BEFORE quilting it. Are the scallops especially large or small?
Good Luck! I would love to know how you solve this challenge.