Can I boot into other OS (such as Ubuntu) from an Android OS tablet that have a USB host? I have the following scenario:

A user have a Android OS tablet that have Android 4.0

The tablet have a full USB port (not the mini USB port)

The user have a thumbdrive that have a boot from USB version Ubuntu.

Is it possible and how do the user goes about booting the USB Ubuntu on an Android OS tablet?

This also apply to any boot from USB version OS.
Short answer: No.
Longer answer: The thumb-drive version of Ubuntu is for x86 processors. There are ways to run Ubuntu in Android, all of them requiring a Ubuntu port with an ARM architecture. Search for Complete Linux Installer in the Play Store for basic stuff. It will rum on top of Android in a virtualized environment, and you can connect to its environment and everything. Just download the image and put it on your flash drive instead.