EDIT: Although launching anything libreoffice-related from the Ubuntu Touch menu seems not to work, I managed to at least start LibreOffice Writer from command line in terminal.
This was the way, maybe it helps someone else who is not a pro at libertine containers:
Install Libertine Tweak Tool
Set all options ON, especially the lirsh option.
Install package libreoffice-writer in the default libertine container (called noble by default) by typing this in a re-launched terminal (to get the lirsh command):
libertine-container-manager install-package -p libreoffice-writer
Type in terminal: lirsh
You will see something starting with (noble)... $
Launch LibreOffice by typing in terminal libreoffice or soffice.
Note that no java (default-jre) was needed, neither libreoffice-java-common. I guess lirsh does some wonders here and sets the paths inside the libertine container. If one knows what lirsh does, it could be used to fix the LibreOffice launcher(s) in the main Ubuntu Touch menu.
At time of testing, clicking on menus File, Edit etc does not produce any visible menu. But it is possible to type some text create a PDF and save it in /tmp folder, which actually is located in the real /tmp folder outside the libertine container. LibreOffice makes Ubuntu Touch unstable and the device restarts Lomiri causing a closure of every app in desktop mode.
