Couple of things you could try:
option 1. (probably the easiest), create a Slitaz LiveCD/LiveUSB, use that to boot another computer that has Internet access, then as root, run the following command:
$tazpkg get-install get-LibreOffice
followed by,
$get-LibreOffice
This will download and install LibreOffice (it will also download and install jre). Once the installation is complete, then as root, run the following commands:
$tazpkg repack LibreOffice
$tazpkg repack java6-jre
This will create two packages (.tazpkg) one for LibreOffice and the other for jre. You can then copy these packages onto a USB drive, and use that to install on your machine with no Internet access (when installing manually, be sure to install java first, then LibreOffice). Note that in order for the repack to work properly, your best bet is to do it right after the installation of LibreOffice finishes, without making any further modifications to the system (installing/removing other stuff).
option 2. Download the .deb package for LibreOffice from http://www.libreoffice.org, move it to the machine you wish to install it on, then convert it into a tazpkg using the following command:
$tazpkg convert LibO_3.4.5_whatever.deb
This will attempt to convert the .deb package to the .tazpkg format. It may or may not work, the convert sometimes fails, especially with large packages. From what I remember, LibreOffice can work without JRE (at least it does on Windows).
option 3. Most painful method, download the source code and compile it.
Probably someone else knows an easier way.