I didn't know about those packs. I looked them up and they are expensive for the size. Maybe they are like the handwarmers and the price would go down as they are mass produced. If they are reusable, my thought would be to just build the sleeping bag out of one big one.
As a side note, I couldn't help but wonder what my camping mates would think of me getting ready to go to bed by sticking a hundred of those Mickey Mouse fever patches all over myself. I get teased enough with my battery powered fan.