Hi,
The solution to display Yelp help entry in right-click menus has been given to me.
The openbox-menu program allows for several flags in menu.xml configuration, to match the environment.
$ openbox-menu --help
Utilisation :
openbox-menu [OPTION...] [menu] - Openbox menu generator 0.3.6.2
Options de l'aide :
-h, --help Affiche les options de l'aide
Options de l'application :
-c, --comment Display comment instead of application name
-i, --noicons Don't display icons in menu
-t, --terminal=cmd Terminal command (default sakura -e)
-g, --gnome Display GNOME entries
-k, --kde Display KDE entries
-x, --xfce Display XFCE entries
-r, --rox Display ROX entries
In the third line of menu.xml changing this:
<menu id="desktop-app" label="Applications" execute="openbox-menu -t 'sakura -e'" />
to this:
<menu id="desktop-app" label="Applications" execute="openbox-menu -g -t 'sakura -e'" />
allows the display of the launcher for the programs who have a desktop file containing "OnlyShowIn=GNOME" as for yelp.desktop.
Now I can fix it for a few installs... in 3 different directories in each and for the next configuration presets.

I also put 2 posts here about desktop files :
Yelp Desktop file "OnlyShowIn" : why ?Desktop files !!! @ù%ù^$£££€€€$$!!!Regards,
Mélodie