After literally minutes of research, I've come to the conclusion that there's no particular pattern for a prayer shawl at all, so you could probably add whatever design features you like. OR... you could just get a shawl pin and call it a day if the aim is to help keep it from slipping off.
The folks who appear to have invented the concept though are on line at http://www.shawlministry.com/. Looks like they have a whole book full of patterns (actually three books full). And they have a contact page, so if anybody could answer the question they probably could.
Since there's no specific pattern for them, you might also look at a poncho or capelet design. There are TONS of those out there, in varying degrees of complexity, and a lot of them are free.