Has anyone been able to get this to work? Should I not be using the template in /cloudvolumes/apps_templates/? Should I be modifying one of the writeable disk templates instead?
Yes. The appstack becomes read only after the provisioning process. If you need to have a read write disk you need to create a writable. Change the writable template (i'd suggest changing the UIA only, saves the hassle of removing the profile part ) and create a writable using that template. It will automatically create a writable with the user name in it so you also know what writable is for what user.
If you were to copy it from 1 datastore to another and import it in another Apppvolumes manager it will be attached to that specific user as well.