How to create custom layout for books?

When creating a photo book, one can select for each page what will be the layout of the photos.

There is a list of possible layouts, sorted by number of photos.

It is often sufficient, but sometimes I want a different layout.

For example, I would like to have three panoramic photos one above the other.

The only possibility that I have seen described on Internet is to create a new image with an external photo editor. This image includes the three photos with the desired layout, and is put in the photo book using the full page layout.

I was wondering: there is probably somewhere in /Applications/Photos.app or in /System/Library a collections of files that describe the existing proposed layouts. And it might be possible to extend the possibilities of Photos by adding new layouts.

Did anyone look into the way Photos implements page layouts for books?