Currently if you want to have a custom UI for each application you need to edit the NotificationLocalization.xml after creation. If you want to have a custom UI for each update then you need to edit the default before publishing each update. It would be useful to be able to save a custom UI against each individual app and update.
Came here to submit this idea. We have an app which requires reboot after getting updated and would like a custom message letting the user know they will need to reboot immediately after the application updates.