You could build your own linux kernel from kernel.org and then install everything by minimal - OpenBox as a DM for example. You probably won't find that kind of distros (or it's just me not aware of them), but you may ask your good linux-advanced friend to do this.
The plan is:
1) compile kernel.
2) install openbox.
3) install libreoffice and all it's dependencies.
4) creare user and strict it to use only libreoffice (if it is strict requirement or you don't want to force use it just install it).
5) create autostart script, configure auto-login if necessary.
6) create distro from your system.
You can find each of this steps on the Internet explained step-by-step.