I'm also looking for a way to create a custom printing template for use with WAB or through an AGOL app template not using ArcGIS Server, has this become available?
Hello,
Currently, in order to have a create a custom printing template, you require ArcGIS for Server.
Here are some documentation links regarding this workflow:
Configure utility services—ArcGIS Online Help | ArcGIS
Please feel free to support the following idea to provide your feedback about this topic:
ArcGIS Idea - Improve available default Map Layouts for Printing in ArcGIS Online
-Kelly
Thank you Kelly