Another reason why a spool won't work: The thread pick up arm, the part that goes up and down with the top thread, would need to be much longer in order for the thread to be relaxed enough to go all the way around the big spool bobbin, and then it needs to pull the thread tight again to get the tension on the stitch correct.
And if you use a chain stitch, you are using three layers of thread, instead of two. And the bends in the thread would cause a problem also, I would think. You would get a whole different fit to your pattern parts with all the extra thread in every seam.