Does anyone know if it is possible to merge an existing AppStack to Guest to make this a permanent/persistent desktop image?
Well, you could assign the stack to the machine, but then you can't have a writeable volume, so make sure you have redirected profiles or some other profile management piece in place. App Volumes + the just-in-time ("Fargo") changes in the next version of View will make it possible for admins to offer persistent experiences with non-persistent configurations. Far easier to manage, in my opinion. You can pretty much do it now, even without JIT desktops.
Once it's possible to assign stacks to both the machine and the user (not necessarily a writeable volume) I'll be a happy guy.