Good question - and when you find the answer please tell the rest of us - now you have me wondering as well.

Well, I've done it in the past, but it's slow going. I can see it in my head, I'll try to explain. Imagine placing a squashed tube(a spring?,a slinky?)around your pressure foot. This would be your basted(squashed)sleeve with your flat fell seam pressed, ready to stitch, facing you. Place the inside(bottom)edge under the needle and start stitching the pressed under edge. As you go, the sleeve moves away from you until you're done and the sleeve is no longer under the foot. I hope this makes sense. I don't know how it's done commercially.