I thought this was an interesting idea to add more interest to the standard scissors truss.
(http://blog.armchairbuilder.com/wp-content/uploads/2011/10/Barrel-Vault-2-1024x768.jpg)
This from an interesting website: http://blog.armchairbuilder.com/2011/11/01/low-costbig-impact-home-details/ (http://blog.armchairbuilder.com/2011/11/01/low-costbig-impact-home-details/)
I assume you would then go to a double layer of 1/4' sheetrock or something like it to make the smooth rounded barrel ceiling?
The article mentions scoring the back of the drywall and that may work on these curves. My understanding is in agreement with yours - and that for (at least) sharper curves you do need multiple layers of the thinner materials.