Perfect, easy and simple!
To change book jacket displayed columns I went into the source folder:
C:\Program Files\Calibre2\app\resources\
and copied the folder named "jacket" into the Calibre configuration folder:
C:\Users\...\AppData\Roaming\calibre
Now in the copied "jacket" folder edit the file template.xhtml to dispaly a custom column as explained in the comments included in the file and it will override the default jacket template
Code:
<!--
In addition you can add code to show the values of custom columns here.
The value is available as _column_name and the title as
_column_name_label. For example, if you have a custom column with
label #genre, you can add it to this template with _genre_label and
_genre. Note that the # is replaced by an underscore. For example
<div><b>{_genre_label}:</b> {_genre}</div>
-->
Side note: the user manual says path for windows is C:\Program Files\Calibre2\resources but my resources path is inside "app" folder as in C:\Program Files\Calibre2\app\resources\
(using Calibre 4.8.0 - windows10)